Expose repo.Info.IsShallowClone

- [ ] Integrate the **[shallow.git](https://github.com/libgit2/libgit2/tree/4811c1500bfaaf8c7e61d712371f3ca429ef2016/tests-clar/resources/shallow.git)** as part of the Libgit2Sharp test repos
- [ ] Bind `git_repository_is_shallow()`
- [ ] Profit!

This fixes #440.