This is the initial integration of the similarity metric into
the `git_diff_find_similar()` code path.  The existing tests all
pass, but the new functionality isn't currently well tested.  The
integration does go through the pluggable metric interface, so it
should be possible to drop in an alternative to the internal
metric that libgit2 implements.

This comes along with a behavior change for an existing interface;
namely, passing two NULLs to git_diff_blobs (or passing NULLs to
git_diff_blob_to_buffer) will now call the file_cb parameter zero
times instead of one time.  I know it's strange that that change
is paired with this other change, but it emerged from some
initialization changes that I ended up making.