Merge branch 'mh/fix-send-email-threaded'

* mh/fix-send-email-threaded:
  send-email: fix a typo in a comment
  send-email: fix threaded mails without chain-reply-to
  add a test for git-send-email for threaded mails without chain-reply-to
  doc/send-email: clarify the behavior of --in-reply-to with --no-thread
  send-email: fix non-threaded mails
  add a test for git-send-email for non-threaded mails