revset: changed spanset to extend _orderedsetmixin

Now spanset can use the lazy min and max methods implementation.

mercurial/revset.py

@@ -2692,7 +2692,7 class _descgeneratorset(_generatorset):
         self._cache[x] = False
         return False
 
-class spanset(object):
+class spanset(_orderedsetmixin):
     """Duck type for baseset class which represents a range of revisions and
     can work lazily and without having all the range in memory