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

See #2735 for the background discussion.
If we wanted things to be more consistent, I think we need to have:
- `git_diff_option_t` -> `git_diff_flags_t` and still use prefix `GIT_DIFF`
- `git_diff_find_t` -> `git_diff_find_flags_t ` and still use prefix `GIT_DIFF_FIND_`

And `git_hashsig_option_t` would become `git_hashsig_flags_t`.

What do you think?