diff -r 652f71b235bf -r ce818cf215dc mercurial/filemerge.py
--- a/mercurial/filemerge.py	Fri Aug 20 21:28:14 2010 -0500
+++ b/mercurial/filemerge.py	Sat Aug 21 10:48:49 2010 -0500
@@ -138,9 +138,6 @@
     if not fco.cmp(fcd): # files identical?
         return None
 
-    if fca == fco: # backwards, use working dir parent as ancestor
-        fca = fcd.parents()[0]
-
     ui = repo.ui
     fd = fcd.path()
     binary = isbin(fcd) or isbin(fco) or isbin(fca)