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

See #2735 for the background discussion.
Are you sure you want to enable `GIT_HASHSIG_ALLOW_SMALL_FILES` unconditionally? The libgit2 behavior doesn't exact match the core Git behavior, I realize, but, for example, core Git contains https://github.com/git/git/blob/master/diffcore-break.c#L68 which will prevent it from splitting a `MODIFIED` pair into `ADD` / `DELETE` items if the file is too small. Disabling this unconditionally will, I think, have the side effect that modifying a 1-line file will always split the pair when `GIT_DIFF_BREAK_REWRITES` is enabled. I think we may want to pair this change with an improvement to the `BREAK_REWRITES` behavior as well.