Add helper methods to git/checkout.go
Add helper methods to git/checkout.go

Bug: skia:7469
Change-Id: Ie62019f019f65d826a20160d949733110a851038
Reviewed-on: https://skia-review.googlesource.com/98660
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot