Fix preprocessing for ref-updated events

The conditional that did a 'git remote update' for ref-updated
events (which otherwise don't affect the Merger) was wrong.

Change-Id: Icb2596df023279442613e10e13104a3621d867d9

zuul/scheduler.py

@@ -563,7 +563,7 @@ class Scheduler(threading.Thread):
             return
 
         # Preprocessing for ref-update events
-        if hasattr(event, 'refspec'):
+        if event.ref:
             # Make sure the local git repo is up-to-date with the remote one.
             # We better have the new ref before enqueuing the changes.
             # This is done before enqueuing the changes to avoid calling an