Replace git command update-ref by a LibGit2Sharp call to UpdateRef()
Refactored PerformRCheckin

- Common checks are still in the base method
- 'Quick' specific code is in _PerformRCheckinQuick
- the non-Quick code is in _PferformRCheckin
GitCommit can now return (forward) Sha of the wrapped commit.
RCheckin now has correct author attribution in quick mode too.

+ Refactored PerformRCheckin to remove duplicate code
Update libgit2sharp.
Eliminate `Process.Start("git", "update-ref", ...)`
Update libgit2sharp to include libgit2/libgit2sharp#421
Use the new refs API.

Thanks @Saaman!!!!
Apply fix for Vs2008
416543 Fix: git-tfs tries to authenticate against all TFS instances in the branch no matter how old they were in use and if they needed at all.
Merge pull request #361 from spraints/update-ref

Replace Process.Start with libgit2sharp. (closes #360)
Merge pull request #362 from EPashkin/fix-git-tfs-authenticates-against-all-tfs-instances_vs2008

Apply fix for Vs2008
Merge pull request #357 from l3m/quick-authors

Correct author attribution for RCheckin Quick