diff -r 6122ad50e38f -r 8dda6f6ff564 mercurial/merge.py
--- a/mercurial/merge.py	Fri Aug 15 23:05:53 2014 +0900
+++ b/mercurial/merge.py	Fri Aug 15 11:48:05 2014 -0500
@@ -1052,7 +1052,7 @@
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
                 pas = [repo[anc] for anc in (sorted(cahs) or [nullid])]
             else:
-                pas = [p1.ancestor(p2, warn=True)]
+                pas = [p1.ancestor(p2, warn=branchmerge)]
 
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)