Revert of scm.GIT.IsValidRevision: Only return True if the given object is a commit (https://codereview.chromium.org/329823002/)

Reason for revert:
Speculative revert to see if this is the cause of auto_roll_test.py to fail in build/ presubmit.

Original issue's description:
> scm.GIT.IsValidRevision: Only return True if the given object is a commit
> 
> BUG=383476
> 
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=276487

TBR=iannucci@chromium.org,maruel@chromium.org,borenet@google.com
NOTREECHECKS=true
NOTRY=true
BUG=383476

Review URL: https://codereview.chromium.org/329393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@276493 0039d316-1c4b-4281-b951-d872f2087c98