[MERGE] Optimize single-file diff/revert/etc

[Aaron Bentley]

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Hi all,

This patch updates Tree._iter_changes so that it performs much faster
when specific files are specified.

It tweaks iter_entries_by_dir to avoid walking portions of the tree that
are not ancestors of specified files.  It does not return entries for
unspecified files.  This is more complicated than just returning the
specified entries, but it ensures that ordering requirements are met.

For the case where a single file is specified, there can be no ordering
requirements, so that entry is simply returned.

Aaron
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