tests/test-narrow-debugcommands.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

  $ . "$TESTDIR/narrow-library.sh"
  $ hg init repo
  $ cd repo
  $ cat << EOF > .hg/store/narrowspec
  > [include]
  > path:foo
  > [exclude]
  > EOF
  $ echo treemanifest >> .hg/requires
  $ echo narrowhg-experimental >> .hg/requires
  $ mkdir -p foo/bar
  $ echo b > foo/f
  $ echo c > foo/bar/f
  $ hg commit -Am hi
  adding foo/bar/f
  adding foo/f
  $ hg debugindex -m
     rev linkrev nodeid       p1           p2
       0       0 14a5d056d75a 000000000000 000000000000
  $ hg debugindex --dir foo
     rev linkrev nodeid       p1           p2
       0       0 e635c7857aef 000000000000 000000000000
  $ hg debugindex --dir foo/
     rev linkrev nodeid       p1           p2
       0       0 e635c7857aef 000000000000 000000000000
  $ hg debugindex --dir foo/bar
     rev linkrev nodeid       p1           p2
       0       0 e091d4224761 000000000000 000000000000
  $ hg debugindex --dir foo/bar/
     rev linkrev nodeid       p1           p2
       0       0 e091d4224761 000000000000 000000000000
  $ hg debugdata -m 0
  foo\x00e635c7857aef92ac761ce5741a99da159abbbb24t (esc)
  $ hg debugdata --dir foo 0
  bar\x00e091d42247613adff5d41b67f15fe7189ee97b39t (esc)
  f\x001e88685f5ddec574a34c70af492f95b6debc8741 (esc)
  $ hg debugdata --dir foo/ 0
  bar\x00e091d42247613adff5d41b67f15fe7189ee97b39t (esc)
  f\x001e88685f5ddec574a34c70af492f95b6debc8741 (esc)
  $ hg debugdata --dir foo/bar 0
  f\x00149da44f2a4e14f488b7bd4157945a9837408c00 (esc)
  $ hg debugdata --dir foo/bar/ 0
  f\x00149da44f2a4e14f488b7bd4157945a9837408c00 (esc)