mq: correct the use of super

The first argument passed to super is supposed to be the class where
it's being used, not self.__class__ - the two can be different when
there's inheritance involved.

hgext/mq.py

@@ -1393,7 +1393,7 b' def reposetup(ui, repo):'
             if self.tagscache:
                 return self.tagscache
 
-            tagscache = super(self.__class__, self).tags()
+            tagscache = super(MqRepo, self).tags()
 
             q = self.mq
             if not q.applied: