Add support for `thread` field in `notmuch show`

[David Bremner]

David Bremner <david at tethera.net> writes:

> Offhand I have no strong objection to someone (who is not me) adding
> this. I think it's important to be aware that thread id's are ephemeral,
> and subject to change e.g. if the database is re-built from
> scratch.

I guess a more common/interesting case is that two threads can merge if
new is indexed.

d