git: support git checkout for a commit hash

git_clone assumes a branch or a tag is passed as the last argument, and
it fails when a commit hash is passed, as in:

timeout -s SIGINT 0 git clone https://github.com/ovn-org/ovn.git
/opt/stack/ovn --branch 36e3ab9b47e93af0599a818e9d6b2930e49473f0
Cloning into '/opt/stack/ovn'...
fatal: Remote branch 36e3ab9b47e93af0599a818e9d6b2930e49473f0 not found
in upstream origin

Change-Id: Id1328d7cba418fa7c227ae9db4fe83c09fd06035
1 file changed