Diff Updates

This PR incorporates some of the latest diff changes from libgit2.

* Expose some missing diff options: `:new_prefix`, `:old_prefix`, `:minimal`, `:oid_abbrev`, `:force_binary`.
* Add `Rugged::Diff#sorted_icase?`.
* Add `Rugged::Diff#each_line`.
* Add `Rugged::Diff::Delta#status_char`.
* Add some missing line origin check methods on `Rugged::Diff::Line`.

Missing:
- [ ] Add tests for `Rugged::Diff#sorted_icase?`.
- [x] Add tests for `Rugged::Diff#each_line`.
- [x] Add tests for `Rugged::Diff::Delta#status_char`.