handle repeated References and In-Reply-To headers
It seems possible for git-send-email(1) to generate repeated
repeated instances of References and In-Reply-To headers,
as evidenced in:
https://public-inbox.org/git/
20161111124541.8216-17-vascomalmeida@sapo.pt/raw
This causes a mismatch between how our search indexer threads
and how our HTML view handles threading. In the future, View.pm
will use the smsg-parsed {references} field and avoid redoing
Email::MIME header parsing.
We will still need to figure out a way to deal with messages
with repeated Message-IDs, at some point, too.