[MERGE] 40% faster partial commits

[Robert Collins]

This makes partial commits walk much less data, with approximately 40%
increase in speed.

The basic approach is to use the iter_entries_by_dir specific_file_ids
parameter, after tweaking it to optionally match the behaviour wanted by
commit.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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