diff -r 2f7186400a07 -r 3605d4e7e618 mercurial/revlog.py
--- a/mercurial/revlog.py	Tue Apr 16 10:08:18 2013 -0700
+++ b/mercurial/revlog.py	Tue Apr 16 10:08:19 2013 -0700
@@ -705,17 +705,12 @@
     def ancestor(self, a, b):
         """calculate the least common ancestor of nodes a and b"""
 
-        # fast path, check if it is a descendant
         a, b = self.rev(a), self.rev(b)
-        start, end = sorted((a, b))
-        if self.descendant(start, end):
-            return self.node(start)
-
-        c = ancestor.ancestor(a, b, self.parentrevs)
-        if c is None:
-            return nullid
-
-        return self.node(c)
+        ancs = ancestor.ancestors(self.parentrevs, a, b)
+        if ancs:
+            # choose a consistent winner when there's a tie
+            return min(map(self.node, ancs))
+        return nullid
 
     def _match(self, id):
         if isinstance(id, int):