Replace git command update-ref by a LibGit2Sharp call to UpdateRef()
Update libgit2sharp.
Eliminate `Process.Start("git", "update-ref", ...)`
Update libgit2sharp to include libgit2/libgit2sharp#421
Use the new refs API.

Thanks @Saaman!!!!
Merge pull request #361 from spraints/update-ref

Replace Process.Start with libgit2sharp. (closes #360)