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?

Note that I only enabled it for `git_diff_find_similar()` not the libgit2 merge code path since @carlosmn mentioned that it could create noise there and that's why not detecting renames of small files was useful.

> I think we may want to pair this change with an improvement to the BREAK_REWRITES behavior as well.

I don't fully understand how rewrites detection is implemented in libgit2 and how this relates to `--break-rewrites` in Git. For instance, why is there even 3 options `GIT_DIFF_FIND_REWRITES`, `GIT_DIFF_BREAK_REWRITES` and `GIT_DIFF_FIND_AND_BREAK_REWRITES` vs a single one to enable rewrite detection?