branchmap: stringify int in a portable way

We actually need a bytes in Python 3, and thanks to our nasty source
loader this will portably do the right thing.

mercurial/branchmap.py

@@ -233,7 +233,7 b' class branchcache(dict):'
     def write(self, repo):
         try:
             f = repo.vfs(_filename(repo), "w", atomictemp=True)
-            cachekey = [hex(self.tipnode), str(self.tiprev)]
+            cachekey = [hex(self.tipnode), '%d' % self.tiprev]
             if self.filteredhash is not None:
                 cachekey.append(hex(self.filteredhash))
             f.write(" ".join(cachekey) + '\n')