Replace use of show-ref with name-rev

Testing showed that show-ref did not accurately determine whether a
git repository contained the given reference.  Testing with name-rev
showed that it did.

Change the HAS_REF determination appropriately to avoid unnecessary
cache updates when using fixed references.

Change-Id: Iece213348516358af84417807f8dd36e959139eb
Merge "Replace use of show-ref with name-rev"