diff -r 6fb3b8c61775 -r 79db6d40bced mercurial/discovery.py
--- a/mercurial/discovery.py	Sat Dec 22 01:34:23 2012 +0100
+++ b/mercurial/discovery.py	Sat Dec 22 01:44:42 2012 +0100
@@ -193,8 +193,9 @@
 
     # D. Update newmap with outgoing changes.
     # This will possibly add new heads and remove existing ones.
-    newmap = dict((branch, heads[1]) for branch, heads in headssum.iteritems()
-                  if heads[0] is not None)
+    newmap = branchmap.branchcache((branch, heads[1])
+                                 for branch, heads in headssum.iteritems()
+                                 if heads[0] is not None)
     branchmap.update(repo, newmap, missingctx)
     for branch, newheads in newmap.iteritems():
         headssum[branch][1][:] = newheads