diff -r 6f26ce7ae175 -r 6f1894d6a6b0 mercurial/filemerge.py
--- a/mercurial/filemerge.py	Mon Apr 19 20:30:31 2010 +0200
+++ b/mercurial/filemerge.py	Mon Apr 19 20:41:53 2010 +0200
@@ -135,6 +135,9 @@
     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)