This implements the basis for diff rename and copy detection,
although it is based on simple SHA comparison right now instead
of using a matching algortihm.  Just as `git_diff_merge` can be
used as a post-pass on diffs to emulate certain command line
behaviors, there is a new API `git_diff_detect` which will
update a diff list in-place, adjusting some deltas to RENAMED
or COPIED state (and also, eventually, splitting MODIFIED deltas
where the change is too large into DELETED/ADDED pairs).

This also adds a new test repo that will hold rename/copy/split
scenarios.  Right now, it just has exact-match rename and copy,
but the tests are written to use tree diffs, so we should be able
to add new test scenarios easily without breaking tests.