over: ensure old, merged {tid} is really gone

We must use the result of link_refs() since it can trigger
merge_threads() and invalidate $old_tid.  In case
merge_threads() isn't triggered, link_refs() will return
$old_tid anyways.

When rethreading and allocating new {tid}, we also must update
the row where the now-expired {tid} came from to ensure only the
new {tid} is seen when reindexing subsequent messages in
history.  Otherwise, every subsequently reindexed+rethreaded
message could end up getting a new {tid}.

Reported-by: Kyle Meyer <kyle@kyleam.com>
Link: https://public-inbox.org/meta/87360nlc44.fsf@kyleam.com/

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