Only call IsValidRevision() when it is a git hash.
Only call IsValidRevision() when it is a git hash.

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@144786 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools