branchmap: `stream_in` write remote branchcache to local one

branchmap: `stream_in` write remote branchcache to local one

The previous code was writing it to a non existent `branchcache` attribute.  We
now write is to the proper `_branchcache` attribute and initialize the
`_branchcachetip` at the same time.

We keep writing it to disk, the previous code had this part right.

--- a/mercurial/localrepo.py	Thu Dec 20 14:45:17 2012 +0100
+++ b/mercurial/localrepo.py	Sat Dec 22 01:34:23 2012 +0100
@@ -2492,12 +2492,12 @@
                 for bheads in rbranchmap.itervalues():
                     rbheads.extend(bheads)
 
-                self.branchcache = rbranchmap
                 if rbheads:
                     rtiprev = max((int(self.changelog.rev(node))
                             for node in rbheads))
-                    branchmap.write(self, self.branchcache,
-                            self[rtiprev].node(), rtiprev)
+                    self._branchcache = rbranchmap
+                    rtipnode = self._branchcachetip = self[rtiprev].node()
+                    branchmap.write(self, self._branchcache, rtipnode, rtiprev)
             self.invalidate()
             return len(self.heads()) + 1
         finally: