notmuch search: Avoid printing duplicate author names.

notmuch search: Avoid printing duplicate author names.

We add a hash to the thread object so that we can detect author names
that have already been added to the list, and avoid adding them
redundantly. This avoids the giant chain of "bugzilla-daemon,
bugzilla-daemon, bugzilla-daemon, bugzilla-daemon, ..." author lists
that we would get otherwise, for example.