Calling IsValidRevision('refs/head/master') will always return True.

TBR=bauerb@chromium.org
BUG=134858
TEST=Tested manually with 'managed': True


Review URL: https://chromiumcodereview.appspot.com/10694022

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@144786 0039d316-1c4b-4281-b951-d872f2087c98