Use libgit2sharp to commit

# Pro

Less shelling out to git.

# Con

Newlines are different in the commit messages, so all the shas change. This isn't a big deal for existing repositories, but it makes it harder for you (who git-tfs cloned the TFS repository before this change) and me (who git-tfs cloned the TFS repository after this change) to share work with each other.

# wip

The unit tests are broken at this point due to the "Con".