mercurial/revlog.py

--- a/mercurial/revlog.py	Sat May 19 19:44:23 2012 -0700
+++ b/mercurial/revlog.py	Sat May 19 19:44:58 2012 -0700
@@ -635,6 +635,10 @@
         return (orderedout, roots, heads)
 
     def headrevs(self):
+        try:
+            return self.index.headrevs()
+        except AttributeError:
+            pass
         count = len(self)
         if not count:
             return [nullrev]