lei q: Support displaying multiple threads

piem-lei-query shows unthreaded results.  From there,
piem-lei-query-thread can be used to show the entire thread for that
result.  This design is driven largely by my use of Notmuch, where I
call notmuch-search and then follow up with a custom variant of
notmuch-tree to show _one_ thread.

However, users familiar with notmuch-tree probably expect to be able
to display _multiple_ threads.  piem-lei-query--thread already returns
a list of threads, so it really just needs to be exposed at the
command level.

Update piem-lei-query-thread to make it handle a general query,
renaming it slightly to make it clearer that the command now supports
displaying multiple threads.  Then, add a wrapper piem-lei-mid-thread
that handles the old "single thread for a given MID" behavior.

Rather than adding another suffix command to the lei-q transient
(piem-lei-query-threads in addition to the existing piem-lei-query), I
considered adding --threads to the transient and then having
piem-lei-query check whether it's in the arguments.  Conceptually, I
dislike that because it conflates threaded _display_ with lei's
--threads behavior that's instead about whether to include other
messages from a match's thread in the results.  Also, it'd mean some
downstream handling of piem-lei-buffer-args (e.g., by
piem-lei-query-show) would be complicated by the need to filter out
--threads.

Note that piem-lei-query-thread no longer sets piem-lei-buffer-mid
because the buffer is no longer tied to a single message ID, which is
okay because, unlike in show buffers, the value isn't actually used.

Suggested-by: Xinglu Chen <public@yoctocell.xyz>
Link: https://inbox.kyleam.com/piem/871r96am1q.fsf@yoctocell.xyz
Message-Id: <20211228022037.206597-4-kyle@kyleam.com>

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