diff -r c6b1be675d3c -r fad5ed0ff997 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Fri Aug 20 17:16:37 2010 -0500
+++ b/mercurial/filemerge.py	Sat Aug 21 10:41:29 2010 -0500
@@ -138,9 +138,6 @@
     if not fco.cmp(fcd.data()): # 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)