Try this:

diff -r 08a0f04b56bd mercurial/copies.py
--- a/mercurial/copies.py	Mon Jan 25 00:05:27 2010 -0600
+++ b/mercurial/copies.py	Mon Jan 25 01:14:41 2010 -0600
@@ -154,7 +154,7 @@
                 break # no merge needed, quit early
             c2 = ctx(of, m2[of])
             cr = related(oc, c2, ca.rev())
-            if of == f or of == c2.path(): # non-divergent
+            if cr and (cr.path() == f or cr.path == c2.path()): # non-divergent
                 copy[f] = of
                 of = None
                 break