py3: convert filtername to str if it's None

I have not called pycompat.bytestr() and rather converted the value there
because I am starting to get concerned about the function call overhead of all
this bytes to str or vice versa convert functions.

Differential Revision: https://phab.mercurial-scm.org/D6042

mercurial/branchmap.py

@@ -360,7 +360,7 b' class branchcache(dict):'
 
         duration = util.timer() - starttime
         repo.ui.log('branchcache', 'updated %s branch cache in %.4f seconds\n',
-                    repo.filtername, duration)
+                    repo.filtername or b'None', duration)
 
         self.write(repo)