generatorset: move iteration code into _iterator

_iterator handles the generator iteration. The `__iter__` method will need
changes to handle ordering-related information.

mercurial/revset.py

@@ -2626,10 +2626,10 b' class generatorset(abstractsmartset):'
         self._finished = False
         if iterasc is not None:
             if iterasc:
-                self.fastasc = self.__iter__
+                self.fastasc = self._iterator
                 self.__contains__ = self._asccontains
             else:
-                self.fastdesc = self.__iter__
+                self.fastdesc = self._iterator
                 self.__contains__ = self._desccontains
 
     def __nonzero__(self):
@@ -2680,6 +2680,9 b' class generatorset(abstractsmartset):'
         return False
 
     def __iter__(self):
+        return self._iterator()
+
+    def _iterator(self):
         if self._finished:
             return iter(self._genlist)