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

elements/source-repositories/extra-data.d/98-source-repositories

@@ -64,7 +64,7 @@ function get_repos_for_element(){
                     mv ${CACHE_PATH}{.tmp,}
                 fi
 
-                HAS_REF=$(git --git-dir=$CACHE_PATH/.git show-ref $REPOREF || true)
+                HAS_REF=$(git --git-dir=$CACHE_PATH/.git name-rev $REPOREF 2>/dev/null || true)
                 if [ -z "$DIB_OFFLINE" -o -z "$HAS_REF" ] ; then
                     echo "Updating cache of $REPOLOCATION in $CACHE_PATH with ref $REPOREF"
                     git --git-dir=$CACHE_PATH/.git fetch --update-head-ok $REPOLOCATION ${REPOREF}:${REPOREF}