Allow git_diff_find_similar() to detect renames and copies of small files

See #2735 for the background discussion.
So it seems like now we're going to bubble up `GIT_EBUFS` to the caller.  Will this not mean that diffs with small files will fail with a `GIT_EBUFS`, wereas we should have swallowed that error before?

Perhaps two tests - one that allows for using small files in rename detection, and one that has the current defaults - would make this change more clear?