generatorset: implement __nonzero__

This is necessary to become a real smartset.

mercurial/revset.py

@@ -2741,6 +2741,11 b' class _generatorset(object):'
         self._genlist = baseset([])
         self._finished = False
 
+    def __nonzero__(self):
+        for r in self:
+            return True
+        return False
+
     def __contains__(self, x):
         if x in self._cache:
             return self._cache[x]