[MERGE] Tree._iter_changes

[Aaron Bentley]

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Hi all,

This bundle implements the Tree._iter_changes API, which is intended to
unify our various change-detection routines.

This will give us a single target for further optimization, and a basis
for supporting file kind changes (e.g. file -> symlink).

In particular, I expect dirstate can provide a very fast implementation,
and we may also be able to optimize revision tree comparison by using
fileids_altered_by_revision_ids.

It's possible that we'll want to tweak it a bit when dirstate lands, so
I've made it a private interface for now.

This patch uses Tree._iter_changes as the underlying implementation for
Tree.changes_from and transform.revert.  If it is accepted, I will
follow-up with changes to commit and possibly merge.

The default implementation goes a long way to avoid repeatedly statting
files.  With it, revert is moderately faster.

bzr.dev:
$ bzr merge ../tree-compare/; time bzr revert
All changes applied successfully.



real    0m1.154s
user    0m1.088s
sys     0m0.060s

tree-compare:
$ bzr merge ../tree-compare/; time ../tree-compare/bzr revert
All changes applied successfully.



real    0m1.113s
user    0m1.060s
sys     0m0.048s

The status benchmarks also show a moderate improvement:
bzr.dev:
bzrlib.benchmarks.bench_startup.StartupBenchmark.test_status   OK
136ms/  257ms
...us.StatusBenchmark.test_no_changes_known_kernel_like_tree   OK
2569ms/58185ms
....StatusBenchmark.test_no_ignored_unknown_kernel_like_tree   OK
1282ms/16247ms
...chmark.test_single_file_no_changes_known_kernel_like_tree   OK
1315ms/39957ms

tree-compare:
bzrlib.benchmarks.bench_startup.StartupBenchmark.test_status   OK
133ms/  324ms
...us.StatusBenchmark.test_no_changes_known_kernel_like_tree   OK
2111ms/20459ms
....StatusBenchmark.test_no_ignored_unknown_kernel_like_tree   OK
1085ms/34728ms
...chmark.test_single_file_no_changes_known_kernel_like_tree   OK
1097ms/29666ms

Aaron
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