reset on origin/HEAD not master

Some repositories might not have a master branch or could use a
different default HEAD. A git reset was made to remotes/master which
would produce a stacktrace.

Fix https://bugs.launchpad.net/zuul/+bug/1083154

Change-Id: Ie9390c0c5dd707d77fbfa8cff2078a9b31fd31ae
Reviewed-on: https://review.openstack.org/17239
Reviewed-by: Antoine Musso <hashar@free.fr>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins

zuul/merger.py

@@ -54,7 +54,8 @@ class Repo(object):
                 continue
             self.repo.create_head(ref.remote_head, ref, force=True)
 
-        self.repo.head.reference = self.repo.heads.master
+        # Reset to remote HEAD (usually origin/master)
+        self.repo.head.reference = origin.refs['HEAD']
         self.repo.head.reset(index=True, working_tree=True)
         self.repo.git.clean('-x', '-f', '-d')