Include ref info in smtp reporter subjects

With the move towards reporting zuul's build dashboard pages we lose
info like the ref/branch that a job ran against when reporting with smtp
(because that info is no longer in the log url). Add the ref info to the
subject lines by default to help make that a little better for our
users.

Change-Id: I9671deb6e1de91832f3830587f90edfe46f06f33

zuul/driver/smtp/smtpreporter.py

@@ -42,7 +42,8 @@ class SMTPReporter(BaseReporter):
             subject = self.config['subject'].format(
                 change=item.change)
         else:
-            subject = "Report for change %s" % item.change
+            subject = "Report for change {change} against {ref}".format(
+                change=item.change, ref=item.change.ref)
 
         self.connection.sendMail(subject, message, from_email, to_email,
                                  zuul_event_id=item.event)