view tests/test-copytrace-heuristics.t @ 38732:be4984261611

merge: mark file gets as not thread safe (issue5933)

In default installs, this has the effect of disabling the thread-based
worker on Windows when manifesting files in the working directory. My
measurements have shown that with revlog-based repositories, Mercurial
spends a lot of CPU time in revlog code resolving file data. This ends
up incurring a lot of context switching across threads and slows down
`hg update` operations when going from an empty working directory to
the tip of the repo.

On mozilla-unified (246,351 files) on an i7-6700K (4+4 CPUs):

before: 487s wall
after:  360s wall (equivalent to worker.enabled=false)
cpus=2: 379s wall

Even with only 2 threads, the thread pool is still slower.

The introduction of the thread-based worker (02b36e860e0b) states that
it resulted in a "~50%" speedup for `hg sparse --enable-profile` and
`hg sparse --disable-profile`. This disagrees with my measurement
above. I theorize a few reasons for this:

1) Removal of files from the working directory is I/O - not CPU - bound
   and should benefit from a thread pool (unless I/O is insanely fast
   and the GIL release is near instantaneous). So tests like `hg sparse
   --enable-profile` may exercise deletion throughput and aren't good
   benchmarks for worker tasks that are CPU heavy.
2) The patch was authored by someone at Facebook. The results were
   likely measured against a repository using remotefilelog. And I
   believe that revision retrieval during working directory updates with
   remotefilelog will often use a remote store, thus being I/O and not
   CPU bound. This probably resulted in an overstated performance gain.

Since there appears to be a need to enable the thread-based worker with
some stores, I've made the flagging of file gets as thread safe
configurable. I've made it experimental because I don't want to formalize
a boolean flag for this option and because this attribute is best
captured against the store implementation. But we don't have a proper
store API for this yet. I'd rather cross this bridge later.

It is possible there are revlog-based repositories that do benefit from
a thread-based worker. I didn't do very comprehensive testing. If there
are, we may want to devise a more proper algorithm for whether to use
the thread-based worker, including possibly config options to limit the
number of threads to use. But until I see evidence that justifies
complexity, simplicity wins.

Differential Revision: https://phab.mercurial-scm.org/D3963

Test for the heuristic copytracing algorithm
============================================

  $ cat >> $TESTTMP/copytrace.sh << '__EOF__'
  > initclient() {
  > cat >> $1/.hg/hgrc <<EOF
  > [experimental]
  > copytrace = heuristics
  > copytrace.sourcecommitlimit = -1
  > EOF
  > }
  > __EOF__
  $ . "$TESTTMP/copytrace.sh"

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > rebase=
  > shelve=
  > EOF

NOTE: calling initclient() set copytrace.sourcecommitlimit=-1 as we want to
prevent the full copytrace algorithm to run and test the heuristic algorithm
without complexing the test cases with public and draft commits.

Check filename heuristics (same dirname and same basename)
----------------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ mkdir dir
  $ echo a > dir/file.txt
  $ hg addremove
  adding a
  adding dir/file.txt
  $ hg ci -m initial
  $ hg mv a b
  $ hg mv -q dir dir2
  $ hg ci -m 'mv a b, mv dir/ dir2/'
  $ hg up -q 0
  $ echo b > a
  $ echo b > dir/file.txt
  $ hg ci -qm 'mod a, mod dir/file.txt'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: 557f403c0afd2a3cf15d7e2fb1f1001a8b85e081
  |   desc: mod a, mod dir/file.txt
  | o  changeset: 928d74bc9110681920854d845c06959f6dfc9547
  |/    desc: mv a b, mv dir/ dir2/
  o  changeset: 3c482b16e54596fed340d05ffaf155f156cda7ee
      desc: initial

  $ hg rebase -s . -d 1
  rebasing 2:557f403c0afd "mod a, mod dir/file.txt" (tip)
  merging b and a to b
  merging dir2/file.txt and dir/file.txt to dir2/file.txt
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/557f403c0afd-9926eeff-rebase.hg
  $ cd ..
  $ rm -rf repo

Make sure filename heuristics do not when they are not related
--------------------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo 'somecontent' > a
  $ hg add a
  $ hg ci -m initial
  $ hg rm a
  $ echo 'completelydifferentcontext' > b
  $ hg add b
  $ hg ci -m 'rm a, add b'
  $ hg up -q 0
  $ printf 'somecontent\nmoarcontent' > a
  $ hg ci -qm 'mode a'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: d526312210b9e8f795d576a77dc643796384d86e
  |   desc: mode a
  | o  changeset: 46985f76c7e5e5123433527f5c8526806145650b
  |/    desc: rm a, add b
  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
      desc: initial

  $ hg rebase -s . -d 1
  rebasing 2:d526312210b9 "mode a" (tip)
  other [source] changed a which local [dest] deleted
  use (c)hanged version, leave (d)eleted, or leave (u)nresolved? u
  unresolved conflicts (see hg resolve, then hg rebase --continue)
  [1]

  $ cd ..
  $ rm -rf repo

Test when lca didn't modified the file that was moved
-----------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo 'somecontent' > a
  $ hg add a
  $ hg ci -m initial
  $ echo c > c
  $ hg add c
  $ hg ci -m randomcommit
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ hg up -q 1
  $ echo b > a
  $ hg ci -qm 'mod a'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
  @  changeset: 9d5cf99c3d9f8e8b05ba55421f7f56530cfcf3bc
  |   desc: mod a, phase: draft
  | o  changeset: d760186dd240fc47b91eb9f0b58b0002aaeef95d
  |/    desc: mv a b, phase: draft
  o  changeset: 48e1b6ba639d5d7fb313fa7989eebabf99c9eb83
  |   desc: randomcommit, phase: draft
  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
      desc: initial, phase: draft

  $ hg rebase -s . -d 2
  rebasing 3:9d5cf99c3d9f "mod a" (tip)
  merging b and a to b
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/9d5cf99c3d9f-f02358cc-rebase.hg
  $ cd ..
  $ rm -rf repo

Rebase "backwards"
------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo 'somecontent' > a
  $ hg add a
  $ hg ci -m initial
  $ echo c > c
  $ hg add c
  $ hg ci -m randomcommit
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ hg up -q 2
  $ echo b > b
  $ hg ci -qm 'mod b'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: fbe97126b3969056795c462a67d93faf13e4d298
  |   desc: mod b
  o  changeset: d760186dd240fc47b91eb9f0b58b0002aaeef95d
  |   desc: mv a b
  o  changeset: 48e1b6ba639d5d7fb313fa7989eebabf99c9eb83
  |   desc: randomcommit
  o  changeset: e5b71fb099c29d9172ef4a23485aaffd497e4cc0
      desc: initial

  $ hg rebase -s . -d 0
  rebasing 3:fbe97126b396 "mod b" (tip)
  merging a and b to a
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/fbe97126b396-cf5452a1-rebase.hg
  $ cd ..
  $ rm -rf repo

Check a few potential move candidates
-------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ mkdir dir
  $ echo a > dir/a
  $ hg add dir/a
  $ hg ci -qm initial
  $ hg mv dir/a dir/b
  $ hg ci -qm 'mv dir/a dir/b'
  $ mkdir dir2
  $ echo b > dir2/a
  $ hg add dir2/a
  $ hg ci -qm 'create dir2/a'
  $ hg up -q 0
  $ echo b > dir/a
  $ hg ci -qm 'mod dir/a'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: 6b2f4cece40fd320f41229f23821256ffc08efea
  |   desc: mod dir/a
  | o  changeset: 4494bf7efd2e0dfdd388e767fb913a8a3731e3fa
  | |   desc: create dir2/a
  | o  changeset: b1784dfab6ea6bfafeb11c0ac50a2981b0fe6ade
  |/    desc: mv dir/a dir/b
  o  changeset: 36859b8907c513a3a87ae34ba5b1e7eea8c20944
      desc: initial

  $ hg rebase -s . -d 2
  rebasing 3:6b2f4cece40f "mod dir/a" (tip)
  merging dir/b and dir/a to dir/b
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/6b2f4cece40f-503efe60-rebase.hg
  $ cd ..
  $ rm -rf repo

Test the copytrace.movecandidateslimit with many move candidates
----------------------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg mv a foo
  $ echo a > b
  $ echo a > c
  $ echo a > d
  $ echo a > e
  $ echo a > f
  $ echo a > g
  $ hg add b
  $ hg add c
  $ hg add d
  $ hg add e
  $ hg add f
  $ hg add g
  $ hg ci -m 'mv a foo, add many files'
  $ hg up -q ".^"
  $ echo b > a
  $ hg ci -m 'mod a'
  created new head

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
  |   desc: mod a
  | o  changeset: 8329d5c6bf479ec5ca59b9864f3f45d07213f5a4
  |/    desc: mv a foo, add many files
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial

With small limit

  $ hg rebase -s 2 -d 1 --config experimental.copytrace.movecandidateslimit=0
  rebasing 2:ef716627c70b "mod a" (tip)
  skipping copytracing for 'a', more candidates than the limit: 7
  other [source] changed a which local [dest] deleted
  use (c)hanged version, leave (d)eleted, or leave (u)nresolved? u
  unresolved conflicts (see hg resolve, then hg rebase --continue)
  [1]

  $ hg rebase --abort
  rebase aborted

With default limit which is 100

  $ hg rebase -s 2 -d 1
  rebasing 2:ef716627c70b "mod a" (tip)
  merging foo and a to foo
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/ef716627c70b-24681561-rebase.hg

  $ cd ..
  $ rm -rf repo

Move file in one branch and delete it in another
-----------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ hg up -q ".^"
  $ hg rm a
  $ hg ci -m 'del a'
  created new head

  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
  @  changeset: 7d61ee3b1e48577891a072024968428ba465c47b
  |   desc: del a, phase: draft
  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |/    desc: mv a b, phase: draft
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial, phase: draft

  $ hg rebase -s 1 -d 2
  rebasing 1:472e38d57782 "mv a b"
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/472e38d57782-17d50e29-rebase.hg
  $ hg up -q c492ed3c7e35dcd1dc938053b8adf56e2cfbd062
  $ ls
  b
  $ cd ..
  $ rm -rf repo

Move a directory in draft branch
--------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ mkdir dir
  $ echo a > dir/a
  $ hg add dir/a
  $ hg ci -qm initial
  $ echo b > dir/a
  $ hg ci -qm 'mod dir/a'
  $ hg up -q ".^"
  $ hg mv -q dir/ dir2
  $ hg ci -qm 'mv dir/ dir2/'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: a33d80b6e352591dfd82784e1ad6cdd86b25a239
  |   desc: mv dir/ dir2/
  | o  changeset: 6b2f4cece40fd320f41229f23821256ffc08efea
  |/    desc: mod dir/a
  o  changeset: 36859b8907c513a3a87ae34ba5b1e7eea8c20944
      desc: initial

  $ hg rebase -s . -d 1
  rebasing 2:a33d80b6e352 "mv dir/ dir2/" (tip)
  merging dir/a and dir2/a to dir2/a
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/a33d80b6e352-fecb9ada-rebase.hg
  $ cd ..
  $ rm -rf server
  $ rm -rf repo

Move file twice and rebase mod on top of moves
----------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ hg mv b c
  $ hg ci -m 'mv b c'
  $ hg up -q 0
  $ echo c > a
  $ hg ci -m 'mod a'
  created new head

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: d413169422167a3fa5275fc5d71f7dea9f5775f3
  |   desc: mod a
  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
  | |   desc: mv b c
  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |/    desc: mv a b
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial
  $ hg rebase -s . -d 2
  rebasing 3:d41316942216 "mod a" (tip)
  merging c and a to c
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/d41316942216-2b5949bc-rebase.hg

  $ cd ..
  $ rm -rf repo

Move file twice and rebase moves on top of mods
-----------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ hg mv b c
  $ hg ci -m 'mv b c'
  $ hg up -q 0
  $ echo c > a
  $ hg ci -m 'mod a'
  created new head
  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: d413169422167a3fa5275fc5d71f7dea9f5775f3
  |   desc: mod a
  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
  | |   desc: mv b c
  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |/    desc: mv a b
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial
  $ hg rebase -s 1 -d .
  rebasing 1:472e38d57782 "mv a b"
  merging a and b to b
  rebasing 2:d3efd280421d "mv b c"
  merging b and c to c
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/472e38d57782-ab8d3c58-rebase.hg

  $ cd ..
  $ rm -rf repo

Move one file and add another file in the same folder in one branch, modify file in another branch
--------------------------------------------------------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg mv a b
  $ hg ci -m 'mv a b'
  $ echo c > c
  $ hg add c
  $ hg ci -m 'add c'
  $ hg up -q 0
  $ echo b > a
  $ hg ci -m 'mod a'
  created new head

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
  |   desc: mod a
  | o  changeset: b1a6187e79fbce851bb584eadcb0cc4a80290fd9
  | |   desc: add c
  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |/    desc: mv a b
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial

  $ hg rebase -s . -d 2
  rebasing 3:ef716627c70b "mod a" (tip)
  merging b and a to b
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/ef716627c70b-24681561-rebase.hg
  $ ls
  b
  c
  $ cat b
  b
  $ rm -rf repo

Merge test
----------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ echo b > a
  $ hg ci -m 'modify a'
  $ hg up -q 0
  $ hg mv a b
  $ hg ci -m 'mv a b'
  created new head
  $ hg up -q 2

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |   desc: mv a b
  | o  changeset: b0357b07f79129a3d08a68621271ca1352ae8a09
  |/    desc: modify a
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial

  $ hg merge 1
  merging b and a to b
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -m merge
  $ ls
  b
  $ cd ..
  $ rm -rf repo

Copy and move file
------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ hg cp a c
  $ hg mv a b
  $ hg ci -m 'cp a c, mv a b'
  $ hg up -q 0
  $ echo b > a
  $ hg ci -m 'mod a'
  created new head

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
  |   desc: mod a
  | o  changeset: 4fc3fd13fbdb89ada6b75bfcef3911a689a0dde8
  |/    desc: cp a c, mv a b
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial

  $ hg rebase -s . -d 1
  rebasing 2:ef716627c70b "mod a" (tip)
  merging b and a to b
  merging c and a to c
  saved backup bundle to $TESTTMP/repo/repo/.hg/strip-backup/ef716627c70b-24681561-rebase.hg
  $ ls
  b
  c
  $ cat b
  b
  $ cat c
  b
  $ cd ..
  $ rm -rf repo

Do a merge commit with many consequent moves in one branch
----------------------------------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ echo b > a
  $ hg ci -qm 'mod a'
  $ hg up -q ".^"
  $ hg mv a b
  $ hg ci -qm 'mv a b'
  $ hg mv b c
  $ hg ci -qm 'mv b c'
  $ hg up -q 1
  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  o  changeset: d3efd280421d24f9f229997c19e654761c942a71
  |   desc: mv b c
  o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |   desc: mv a b
  | @  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
  |/    desc: mod a
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial

  $ hg merge 3
  merging a and c to c
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -qm 'merge'
  $ hg log -G -T 'changeset: {node}\n desc: {desc}, phase: {phase}\n'
  @    changeset: cd29b0d08c0f39bfed4cde1b40e30f419db0c825
  |\    desc: merge, phase: draft
  | o  changeset: d3efd280421d24f9f229997c19e654761c942a71
  | |   desc: mv b c, phase: draft
  | o  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  | |   desc: mv a b, phase: draft
  o |  changeset: ef716627c70bf4ca0bdb623cfb0d6fe5b9acc51e
  |/    desc: mod a, phase: draft
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial, phase: draft
  $ ls
  c
  $ cd ..
  $ rm -rf repo

Test shelve/unshelve
-------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg add a
  $ hg ci -m initial
  $ echo b > a
  $ hg shelve
  shelved as default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg mv a b
  $ hg ci -m 'mv a b'

  $ hg log -G -T 'changeset: {node}\n desc: {desc}\n'
  @  changeset: 472e38d57782172f6c6abed82a94ca0d998c3a22
  |   desc: mv a b
  o  changeset: 1451231c87572a7d3f92fc210b4b35711c949a98
      desc: initial
  $ hg unshelve
  unshelving change 'default'
  rebasing shelved changes
  merging b and a to b
  $ ls
  b
  $ cat b
  b
  $ cd ..
  $ rm -rf repo

Test full copytrace ability on draft branch
-------------------------------------------

File directory and base name changed in same move
  $ hg init repo
  $ initclient repo
  $ mkdir repo/dir1
  $ cd repo/dir1
  $ echo a > a
  $ hg add a
  $ hg ci -qm initial
  $ cd ..
  $ hg mv -q dir1 dir2
  $ hg mv dir2/a dir2/b
  $ hg ci -qm 'mv a b; mv dir1 dir2'
  $ hg up -q '.^'
  $ cd dir1
  $ echo b >> a
  $ cd ..
  $ hg ci -qm 'mod a'

  $ hg log -G -T 'changeset {node}\n desc {desc}, phase: {phase}\n'
  @  changeset 6207d2d318e710b882e3d5ada2a89770efc42c96
  |   desc mod a, phase: draft
  | o  changeset abffdd4e3dfc04bc375034b970299b2a309a1cce
  |/    desc mv a b; mv dir1 dir2, phase: draft
  o  changeset 81973cd24b58db2fdf18ce3d64fb2cc3284e9ab3
      desc initial, phase: draft

  $ hg rebase -s . -d 1 --config experimental.copytrace.sourcecommitlimit=100
  rebasing 2:6207d2d318e7 "mod a" (tip)
  merging dir2/b and dir1/a to dir2/b
  saved backup bundle to $TESTTMP/repo/repo/.hg/strip-backup/6207d2d318e7-1c9779ad-rebase.hg
  $ cat dir2/b
  a
  b
  $ cd ..
  $ rm -rf repo

Move directory in one merge parent, while adding file to original directory
in other merge parent. File moved on rebase.

  $ hg init repo
  $ initclient repo
  $ mkdir repo/dir1
  $ cd repo/dir1
  $ echo dummy > dummy
  $ hg add dummy
  $ cd ..
  $ hg ci -qm initial
  $ cd dir1
  $ echo a > a
  $ hg add a
  $ cd ..
  $ hg ci -qm 'hg add dir1/a'
  $ hg up -q '.^'
  $ hg mv -q dir1 dir2
  $ hg ci -qm 'mv dir1 dir2'

  $ hg log -G -T 'changeset {node}\n desc {desc}, phase: {phase}\n'
  @  changeset e8919e7df8d036e07b906045eddcd4a42ff1915f
  |   desc mv dir1 dir2, phase: draft
  | o  changeset 7c7c6f339be00f849c3cb2df738ca91db78b32c8
  |/    desc hg add dir1/a, phase: draft
  o  changeset a235dcce55dcf42034c4e374cb200662d0bb4a13
      desc initial, phase: draft

  $ hg rebase -s . -d 1 --config experimental.copytrace.sourcecommitlimit=100
  rebasing 2:e8919e7df8d0 "mv dir1 dir2" (tip)
  saved backup bundle to $TESTTMP/repo/repo/.hg/strip-backup/e8919e7df8d0-f62fab62-rebase.hg
  $ ls dir2
  a
  dummy
  $ rm -rf repo

Testing the sourcecommitlimit config
-----------------------------------

  $ hg init repo
  $ initclient repo
  $ cd repo
  $ echo a > a
  $ hg ci -Aqm "added a"
  $ echo "more things" >> a
  $ hg ci -qm "added more things to a"
  $ hg up 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo b > b
  $ hg ci -Aqm "added b"
  $ mkdir foo
  $ hg mv a foo/bar
  $ hg ci -m "Moved a to foo/bar"
  $ hg log -G -T 'changeset {node}\n desc {desc}, phase: {phase}\n'
  @  changeset b4b0f7880e500b5c364a5f07b4a2b167de7a6fb0
  |   desc Moved a to foo/bar, phase: draft
  o  changeset 5f6d8a4bf34ab274ccc9f631c2536964b8a3666d
  |   desc added b, phase: draft
  | o  changeset 8b6e13696c38e8445a759516474640c2f8dddef6
  |/    desc added more things to a, phase: draft
  o  changeset 9092f1db7931481f93b37d5c9fbcfc341bcd7318
      desc added a, phase: draft

When the sourcecommitlimit is small and we have more drafts, we use heuristics only

  $ hg rebase -s 8b6e13696 -d .
  rebasing 1:8b6e13696c38 "added more things to a"
  other [source] changed a which local [dest] deleted
  use (c)hanged version, leave (d)eleted, or leave (u)nresolved? u
  unresolved conflicts (see hg resolve, then hg rebase --continue)
  [1]

But when we have "sourcecommitlimit > (no. of drafts from base to c1)", we do
fullcopytracing

  $ hg rebase --abort
  rebase aborted
  $ hg rebase -s 8b6e13696 -d . --config experimental.copytrace.sourcecommitlimit=100
  rebasing 1:8b6e13696c38 "added more things to a"
  merging foo/bar and a to foo/bar
  saved backup bundle to $TESTTMP/repo/repo/repo/.hg/strip-backup/8b6e13696c38-fc14ac83-rebase.hg
  $ cd ..
  $ rm -rf repo