revset: changed _iterator() in addset to use the generated list when available

Now when all the elements have been generated, the iterator will just use the
generated list instead of going through all the elements again.

mercurial/revset.py

@@ -2353,8 +2353,9 b' class _addset(object):'
         return self._iter
 
     def __iter__(self):
-        for r in self._iterator():
-            yield r
+        if self._genlist:
+            return iter(self._genlist)
+        return iter(self._iterator())
 
     def __contains__(self, x):
         return x in self._r1 or x in self._r2