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

See #2735 for the background discussion.
Actually looking more into this the current nomenclature is inconsistent:
- flags for `git_diff_options` are of type `git_diff_option_t` and use prefix `GIT_DIFF_`
- flags for `git_diff_find_options` are of type `git_diff_find_t` and use prefix `GIT_DIFF_FIND_`
- flags for `git_diff_format_email_options` are of type `git_diff_format_email_flags_t` and use prefix `GIT_DIFF_FORMAT_EMAIL_`
- flags for `git_merge_file_options` are of type `git_merge_file_flags_t` and use prefix `GIT_MERGE_FILE_`