py3: fix exception in pull when several things happen to a bookmark

Specifically, when `changes` is:

  [(b'@upstream-committed', None, <function ui.status>, b'updating bookmark @upstream-committed\n'),
   (b'@upstream-committed', binary-node, <function ui.warn>, b'divergent bookmark @ stored as @upstream-committed\n')]

sorting the list raises:

  TypeError: '<' not supported between instances of 'bytes' and 'NoneType'

Differential Revision: https://phab.mercurial-scm.org/D8523

mercurial/bookmarks.py

@@ -754,7 +754,8 b' def updatefromremote(ui, repo, remotemar'
     if changed:
         tr = trfunc()
         changes = []
-        for b, node, writer, msg in sorted(changed):
+        key = lambda t: (t[0], t[1] or b'')
+        for b, node, writer, msg in sorted(changed, key=key):
             changes.append((b, node))
             writer(msg)
         localmarks.applychanges(repo, tr, changes)