contrib/nmbug/nmbug-status: combine thread messages

Newer patch email containing In-Reply-To: to an email sent some time ago
(i.e. to a "thread") was not visible in that "thread" in patch view when
another patch "thread" was submitted in between. This change collects
all messages in every (notmuch-created) thread together before printing
all these threads out in a patch view.

Thanks to Ethan Glasser-Camp for initial review and suggestions with
code examples.