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

See #2735 for the background discussion.
Not really. This test was written assuming `git_hashsig_option_t` was a sequential enum not a bit field enum so the `for()` loop right after will not test things correctly anymore (even silently pass instead of failing) if these 3 constants don't have the sequential values 0,1,2. So I added this extra check rather than rewriting the entire test and making the diff larger.