revset: added set method to addset to duck type generatorset

Since this class is only going to be used inside revset.py (it does not duck
type baseset) it needs to duck type only a few more methods for the next
patches.

mercurial/revset.py

@@ -2340,6 +2340,9 @@ class _addset(object):
     def __contains__(self, x):
         return x in self._r1 or x in self._r2
 
+    def set(self):
+        return self
+
 class _generatorset(object):
     """Wrap a generator for lazy iteration