tests/test-narrow-archive.t

cborutil: change buffering strategy

Profiling revealed that we were spending a lot of time on the
line that was concatenating the old buffer with the incoming data
when attempting to decode long byte strings, such as manifest
revisions.

Essentially, we were feeding N chunks of size len(X) << len(Y) into
decode() and continuously allocating a new, larger buffer to hold
the undecoded input. This created substantial memory churn and
slowed down execution.

Changing the code to aggregate pending chunks in a list until we
have enough data to fully decode the next atom makes things much
more efficient.

I don't have exact data, but I recall the old code spending >1s
on manifest fulltexts from the mozilla-unified repo. The new code
doesn't significantly appear in profile output.

Differential Revision: https://phab.mercurial-scm.org/D4854

Make a narrow clone then archive it
  $ . "$TESTDIR/narrow-library.sh"

  $ hg init master
  $ cd master

  $ for x in `$TESTDIR/seq.py 3`; do
  >   echo $x > "f$x"
  >   hg add "f$x"
  >   hg commit -m "Add $x"
  > done

  $ hg serve -a localhost -p $HGPORT1 -d --pid-file=hg.pid
  $ cat hg.pid >> "$DAEMON_PIDS"

  $ cd ..
  $ hg clone --narrow --include f1 --include f2 http://localhost:$HGPORT1/ narrowclone1
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 3 changesets with 2 changes to 2 files
  new changesets * (glob)
  updating to branch default
  2 files updated, 0 files merged, 0 files removed, 0 files unresolved

The tar should only contain f1 and f2
  $ cd narrowclone1
  $ hg archive -t tgz repo.tgz
  $ tar tfz repo.tgz
  repo/f1
  repo/f2