revlog: if start is nullrev, end is always a descendant stable

revlog: if start is nullrev, end is always a descendant

--- a/mercurial/revlog.py	Mon Nov 08 01:41:42 2010 +0100
+++ b/mercurial/revlog.py	Sun Nov 07 18:16:07 2010 +0900
@@ -869,6 +869,8 @@
         return c
 
     def descendant(self, start, end):
+        if start == nullrev:
+            return True
         for i in self.descendants(start):
             if i == end:
                 return True