--- a/mercurial/commands.py	Tue Sep 09 17:16:24 2014 -0400
+++ b/mercurial/commands.py	Tue Aug 19 01:13:10 2014 +0200
@@ -454,7 +454,7 @@
     node = scmutil.revsingle(repo, rev).node()
 
     op1, op2 = repo.dirstate.parents()
-    if node not in repo.changelog.commonancestorsheads(op1, node):
+    if not repo.changelog.isancestor(node, op1):
         raise util.Abort(_('cannot backout change that is not an ancestor'))
 
     p1, p2 = repo.changelog.parents(node)

--- a/mercurial/revlog.py	Tue Sep 09 17:16:24 2014 -0400
+++ b/mercurial/revlog.py	Tue Aug 19 01:13:10 2014 +0200
@@ -745,6 +745,13 @@
             ancs = ancestor.commonancestorsheads(self.parentrevs, a, b)
         return map(self.node, ancs)
 
+    def isancestor(self, a, b):
+        """return True if node a is an ancestor of node b
+
+        The implementation of this is trivial but the use of
+        commonancestorsheads is not."""
+        return a in self.commonancestorsheads(a, b)
+
     def ancestor(self, a, b):
         """calculate the least common ancestor of nodes a and b"""