templater: implement __len__ for _hybrid

Objects of class _hybrid are returned by such template keywords as children,
bookmarks, tags and others, and also by revset() template function. They are
representing "list of strings" (as hg help template says) for use in templates.
So it would be logical to implement a handy way to count the number of strings
in such list, and that's what __len__ method does.

mercurial/templatekw.py

@@ -26,6 +26,8 b' class _hybrid(object):'
     def __call__(self):
         for x in self.values:
             yield x
+    def __len__(self):
+        return len(self.values)
 
 def showlist(name, values, plural=None, element=None, **args):
     if not element: