Fix checkout when preparing a ref

The new checkout method was relying on out of date information
stored in the remote which was not being updated by the fetch
command.  Instead, just checkout FETCH_HEAD using git directly
so that the remote does not need to be kept up to date.

Also, reset and clean _before_ checking out, since that's supposed
to clean up from messy merges, etc.

Change-Id: Ie47b675512edc36e8aeb9b537ca945ad8d07b780

zuul/merger.py

@@ -63,15 +63,13 @@ class Repo(object):
     def checkout(self, ref):
         self._ensure_cloned()
         self.log.debug("Checking out %s" % ref)
-        self.log.debug(repr(ref))
+        self.repo.head.reset(index=True, working_tree=True)
+        self.repo.git.clean('-x', '-f', '-d')
         if self.repo.re_hexsha_only.match(ref):
-            self.repo.head.reference = ref
+            self.repo.git.checkout(ref)
         else:
             self.fetch(ref)
-            self.repo.head.reference = \
-                self.repo.remotes.origin.refs[ref].commit.hexsha
-        self.repo.head.reset(index=True, working_tree=True)
-        self.repo.git.clean('-x', '-f', '-d')
+            self.repo.git.checkout("FETCH_HEAD")
 
     def cherryPick(self, ref):
         self._ensure_cloned()