revset: changed addset to extend _orderedsetmixin

Now _addset can use the lazy min and max implementation.

mercurial/revset.py

@@ -2421,7 +2421,7 b' class orderedlazyset(_orderedsetmixin, l'
         self._subset.reverse()
         self._ascending = not self._ascending
 
-class _addset(object):
+class _addset(_orderedsetmixin):
     """Represent the addition of two sets
 
     Wrapper structure for lazily adding two structures without losing much