branchmap: read and write key part related to filtered revision

Now that we have a third part for the cache key we need to write and read it on
disk. It is only written when there is filtered revision. This keep the format
compatible with older version.

Notes that, at this state, filtered repository does not use any disk caches yet.

mercurial/branchmap.py

@@ -18,9 +18,14 b' def read(repo):'
         return branchcache()
 
     try:
-        last, lrev = lines.pop(0).split(" ", 1)
+        cachekey = lines.pop(0).split(" ", 2)
+        last, lrev = cachekey[:2]
         last, lrev = bin(last), int(lrev)
-        partial = branchcache(tipnode=last, tiprev=lrev)
+        filteredhash = None
+        if len(cachekey) > 2:
+            filteredhash = bin(cachekey[2])
+        partial = branchcache(tipnode=last, tiprev=lrev,
+                              filteredhash=filteredhash)
         if not partial.validfor(repo):
             # invalidate the cache
             raise ValueError('tip differs')
@@ -113,7 +118,10 b' class branchcache(dict):'
     def write(self, repo):
         try:
             f = repo.opener("cache/branchheads", "w", atomictemp=True)
-            f.write("%s %s\n" % (hex(self.tipnode), self.tiprev))
+            cachekey = [hex(self.tipnode), str(self.tiprev)]
+            if self.filteredhash is not None:
+                cachekey.append(hex(self.filteredhash))
+            f.write(" ".join(cachekey) + '\n')
             for label, nodes in self.iteritems():
                 for node in nodes:
                     f.write("%s %s\n" % (hex(node), encoding.fromlocal(label)))