v1: fix for threading of replies to ghost messages

[Gregor Zattler]

Hi David,
* David Bremner <david at tethera.net> [2018-07-21; 08:37]:
> See the thread at id:87efgmmysi.fsf at len.workgroup for discussion.
>
> This series finally bites the bullet and uses the References header
> for threading.
>
> Please test this series, there are lots of tricky cases with threading.

I applied the patches and looked with notmuch-emacs for threads
which before would probably show wrong ordering of messages.  The
half a dozen threads were all shown in the correct order with
respect to parents and children, but I found from those not
disclosable RT ticket mails (I mentioned in above referenced
thread):

- Threads where the "Resolved" message was shown first but it's
  References: header contains only a fake Message-Id: (which is
  strange: why does notmuch show this resolved-mail in this
  thread?).  


- one thread where two children of one message were shown with
  correct (meaning: same) indentation but the older one before
  (over) the newer one.


I'll ask if I may disclose this threads if this it would be
helpful for diagnosis.


Thanks for your attention and work regarding my problem report
with notmuch.

Ciao; Gregor