Fix resetting repos without new changes.

This brings the command in line with gerrit-git-prep.

Change-Id: I5f40403e506d0b759b77a5cc27b0eb14d3cfa085

devstack-vm-gate-wrap.sh

@@ -104,7 +104,7 @@ function setup_workspace {
 	      exit 1
 	  fi
 	  git checkout $BRANCH
-	  git reset --hard
+	  git reset --hard remotes/origin/$BRANCH
 	  git clean -x -f -d -q
       fi