fix up git check
fix up git check

* fix for when the only commit is initial commit
* refactor to use rev-list instead of parsing log
* remove --depth 10; problematic when ref is 10+ commits ago

starkandwayne/git-multibranch-resource,barthy1/git-resource,zachgersh/git-resource,ivojinovic/pipeline-git-branch-resource,concourse/git-resource,vito/git-resource,StartupMakers/ssh-resource,pivotaltracker/git-branches-resource