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
Merge pull request #357 from l3m/quick-authors

Correct author attribution for RCheckin Quick