mail: add a TODO about proper mbox locking

Per @nbjoerg in IRC, the mbox isn't locked correctly.

Differential Revision: https://phab.mercurial-scm.org/D10505

mercurial/mail.py

@@ -220,6 +220,7 b' def _sendmail(ui, sender, recipients, ms'
 
 def _mbox(mbox, sender, recipients, msg):
     '''write mails to mbox'''
+    # TODO: use python mbox library for proper locking
     with open(mbox, b'ab+') as fp:
         # Should be time.asctime(), but Windows prints 2-characters day
         # of month instead of one. Make them print the same thing.