tests/test-remotefilelog-keepset.t

dirstate-tree: Avoid BTreeMap double-lookup when inserting a dirstate entry

The child nodes of a given node in the tree-shaped dirstate are kept in a
`BTreeMap` where keys are file names as strings. Finding or inserting a value
in the map takes `O(log(n))` string comparisons, which adds up when constructing
the tree.

The `entry` API allows finding a "spot" in the map that may or may not be
occupied and then access that value or insert a new one without doing map
lookup again. However the current API is limited in that calling `entry`
requires an owned key (and so a memory allocation), even if it ends up not
being used in the case where the map already has a value with an equal key.

This is still a win, with 4% better end-to-end time for `hg status` measured
here with hyperfine:

```
Benchmark #1: ../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):      1.337 s ±  0.018 s    [User: 892.9 ms, System: 437.5 ms]
  Range (min … max):    1.316 s …  1.373 s    10 runs

Benchmark #2: ./hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1
  Time (mean ± σ):      1.291 s ±  0.008 s    [User: 853.4 ms, System: 431.1 ms]
  Range (min … max):    1.283 s …  1.309 s    10 runs

Summary
  './hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1' ran
    1.04 ± 0.02 times faster than '../hg2/hg status -R $REPO --config=experimental.dirstate-tree.in-memory=1'
```

* ./hg is this revision
* ../hg2/hg is its parent
* $REPO is an old snapshot of mozilla-central

Differential Revision: https://phab.mercurial-scm.org/D10550

#require no-windows

  $ . "$TESTDIR/remotefilelog-library.sh"

  $ hg init master
  $ cd master
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > server=True
  > serverexpiration=-1
  > EOF
  $ echo x > x
  $ hg commit -qAm x
  $ echo y > y
  $ hg commit -qAm y
  $ echo z > z
  $ hg commit -qAm z
  $ cd ..

  $ hgcloneshallow ssh://user@dummy/master shallow -q
  3 files fetched over 1 fetches - (3 misses, 0.00% hit ratio) over *s (glob)

# Compute keepset for 0th and 2nd commit, which implies that we do not process
# the 1st commit, therefore we diff 2nd manifest with the 0th manifest and
# populate the keepkeys from the diff
  $ cd shallow
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > pullprefetch=0+2
  > EOF
  $ hg debugkeepset

# Compute keepset for all commits, which implies that we only process deltas of
# manifests of commits 1 and 2 and therefore populate the keepkeys from deltas
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > pullprefetch=all()
  > EOF
  $ hg debugkeepset