diff -r caf10970950e -r 6f1d1ed3e19a mercurial/merge.py
--- a/mercurial/merge.py	Wed Jun 16 00:22:10 2010 +0200
+++ b/mercurial/merge.py	Sun Jun 20 14:21:56 2010 -0500
@@ -467,7 +467,8 @@
             raise util.Abort(_("outstanding uncommitted merges"))
         if branchmerge:
             if pa == p2:
-                raise util.Abort(_("can't merge with ancestor"))
+                raise util.Abort(_("merging with a working directory ancestor"
+                                   " has no effect"))
             elif pa == p1:
                 if p1.branch() != p2.branch():
                     fastforward = True