diff -r 1b05ffce47bd -r f0eeb9b3444a mercurial/discovery.py
--- a/mercurial/discovery.py	Mon Dec 24 02:22:04 2012 +0100
+++ b/mercurial/discovery.py	Sat Dec 22 17:08:15 2012 +0100
@@ -196,7 +196,7 @@
     newmap = branchmap.branchcache((branch, heads[1])
                                  for branch, heads in headssum.iteritems()
                                  if heads[0] is not None)
-    branchmap.update(repo, newmap, missingctx)
+    newmap.update(repo, missingctx)
     for branch, newheads in newmap.iteritems():
         headssum[branch][1][:] = newheads
     return headssum