generatorset: implement __len__

It was the only smartset class without a `__len__` implementation.

mercurial/revset.py

@@ -2776,6 +2776,11 b' class generatorset(abstractsmartset):'
             self.fastasc = asc.__iter__
             self.fastdesc = asc.__reversed__
 
+    def __len__(self):
+        for x in self._consumegen():
+            pass
+        return len(self._genlist)
+
     def sort(self, reverse=False):
         self._ascending = not reverse