bookmarks: write bookmarks file deterministically

Bookmarks are internally stored as an unsorted dictionary. Let's at
least write out the bookmarks file in a deterministic order so it is
easier to test, diff, etc.

Differential Revision: https://phab.mercurial-scm.org/D2469

mercurial/bookmarks.py

@@ -179,7 +179,7 b' class bmstore(dict):'
         self._aclean = True
 
     def _write(self, fp):
-        for name, node in self.iteritems():
+        for name, node in sorted(self.iteritems()):
             fp.write("%s %s\n" % (hex(node), encoding.fromlocal(name)))
         self._clean = True
         self._repo.invalidatevolatilesets()