When use origin/master, if we re-installed the devstack with a
RECLONE=True, an error will happen with the error message:
[ERROR] /opt/stack/devstack/functions-common:560 origin/master is neither branch nor tag
Use the branch name without 'origin/' works the same.
Similar change: https://review.openstack.org/#/c/333826
Change-Id: I16f6bd28ad27d4e957c226ed3fceb8881af6e84f