baseset: access `_set` directly for containment check

The `.set()` method is going away.

mercurial/revset.py

@@ -2352,7 +2352,7 b' class baseset(abstractsmartset):'
 
     @util.propertycache
     def __contains__(self):
-        return self.set().__contains__
+        return self._set.__contains__
 
     def __nonzero__(self):
         return bool(self._list)