searchidx: fix ghost root vivification

searchidx: fix ghost root vivification

Due to the asynchronous nature of SMTP, it is possible for the
root message of a thread (with no References/In-Reply-To)
to arrive last in a series.  We must preserve the thread_id
of the ghost message in this case, as we do when vivifiying
non-root ghosts.

Otherwise, this causes threads to be broken when the root
arrives last.