lei q: do not collapse threads with `-tt'

lei q: do not collapse threads with `-tt'

While having Xapian collapse threads is an easy way to reduce
the amount of deduplication work we need to do when writing
out threads; we can't rely on it when using `lei q -tt` since
that needs to flag all hits.

Reported-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Link: https://public-inbox.org/git/Y+pgBmj0jxR+cVkD@mail.gmail.com/