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

See #2735 for the background discussion.
My understanding of the conventions for the libgit2 APIs is that `options` means a struct e.g. `git_diff_find_options`.

Independently of this, I think the name for this enum must change as it's interpretation is changing (from value to bitfield). I can rename it `git_hashsig_options_t` (plural), I don't really care.