contrib: pick: fix refresh result

The function notmuch-pick-refresh-result (used to update tag changes)
was not quite correct: sometimes it got the choice between the subject
and " ..." wrong. This was always true but the new code often calls
this (when opening a message in the message pane to remove the unread
tag) while the async pick process is still running and this caused
mistakes which made the tests fail.

Thus we store the previous subject with the message.