Port our Git model to LibGit2 and LibGit2Sharp

There are three things to be pushed remaining for this PR to be mergeable:
* [ ] LibGit2Sharp to merge https://github.com/libgit2/libgit2sharp/pull/762 so I can bump. This fixes a problem in the native libs with authentication.
* [ ] Linux build script - I have a commit for this that's after a local commit bump to my libgit2sharp module. Waiting on above.
* [ ] Progress monitoring - I have a commit that's on top of the linux build script.