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copies: choose target directory based on longest match

Authored by martinvonz.

Description

copies: choose target directory based on longest match

If one side of a merge renames dir1/ to dir2/ and the subdirectory
dir1/subdir1/ to dir2/subdir2/, and the other side of the merge
adds a file in dir1/subdir1/, we should clearly move that into
dir2/subdir2/. We already detect the directories correctly before
this patch, but we iterate over them in arbitrary order. That results
in the new file sometimes ending up in dir2/subdir1/ instead. This
patch fixes it by iterating over the source directories by visiting
subdirectories first. That's achieved by simply iterating over them in
reverse lexicographical order.

Without the fix, the test case still passes on Python 2 but fails on
Python 3. It depends on the iteration order of the dict. I did not
look into how it's built up and why it behaved differently before the
fix. I could probably have gotten it to fail on Python 2 as well by
choosing different directory names.

Differential Revision: https://phab.mercurial-scm.org/D10115