Use Ref as Revision

Change to use the short ref for the proposed revision instead
of Gerrit Ref Spec.  This allows the job to succeed even if
there is no refspec, such as when a new branch is created.

Change-Id: Icec5615f8e01f33507dc87765a240a3de2489e09
Signed-off-by: beierlm <mark.beierl@canonical.com>