transaction: run _writejournal unfiltered

The function use the length of the repository, something affected by filtering.
It seems better to use the unfiltered length here.

Credit for finding this goes to Durham Goode.

mercurial/localrepo.py

@@ -1120,6 +1120,7 @@ class localrepository(object):
     def undofiles(self):
         return [(vfs, undoname(x)) for vfs, x in self._journalfiles()]
 
+    @unfilteredmethod
     def _writejournal(self, desc):
         self.dirstate.savebackup(None, prefix='journal.')
         self.vfs.write("journal.branch",