follow lg2 master

I set this PR up mainly because of an major change in the inner workings of `Git::CloneOperation`. It now uses the available `git_clone_options`.

:checkered_flag: TODO's:
- [x] After merging the `Git::String` wrapper (whatever its final name will be). The changes must be taken here as well.
- [x] Implement wrappers for `git_checkout_options` and `git_clone_options`