Give props to Hugo Bonacci

EnumExtensions was based on a Stack Overflow answer by Hugo Bonacci.
Reached out to him to get permission and he kindly gave it to us on
January 4 2013.  Adding a link back in this class to ensure that he
gets the proper credit.
Rename Enum.Has<T> to Compat.Enum.HasFlag<T>
Update libgit2 binaries to 7dfc5c3

https://github.com/libgit2/libgit2/compare/16e6cee...7dfc5c3
Move clone related tests into a separate fixture
Add local cloning test coverage
Improve perf and correctness of UTF-8 marshaling
Introduce Repository.Refs.Head