--- a/mercurial/merge.py	Sat Feb 09 21:24:24 2013 +0000
+++ b/mercurial/merge.py	Sat Feb 09 21:24:36 2013 +0000
@@ -200,6 +200,8 @@
         pa = wctx
     elif pa == p2: # backwards
         pa = wctx.p1()
+    elif not branchmerge and not wctx.dirty(missing=True):
+        pass
     elif pa and repo.ui.configbool("merge", "followcopies", True):
         ret = copies.mergecopies(repo, wctx, p2, pa)
         copy, movewithdir, diverge, renamedelete = ret