[IMP] runbot_merge: remove ref update while squashing/rebasing

It should be unnecessary: creating commits directly does not update
the ref (hence 2b1cd83b07) and we're
forcefully setting the ref afterwards, either resetting it to the
original head (for rebase) or updating it to the commits we've just
created (for squash).

runbot_merge/github.py

@@ -124,7 +124,6 @@ class GH(object):
             assert len(c['parents']) == 1, "can't rebase commits with more than one parent"
             tmp_msg = 'temp rebasing PR %s (%s)' % (pr, c['sha'])
             c['new_tree'] = self.merge(c['sha'], dest, tmp_msg)['tree']['sha']
-        self.set_ref(dest, original_head)
 
         prev = original_head
         for c in commits: