test thread breakage when messages are removed and re-added

This test (T590-thread-breakage.sh) has known-broken subtests.

If you have a two-message thread where message "B" is in-reply-to "A",
notmuch rightly sees this as a single thread.

But if you:

 * remove "A" from the message store
 * run "notmuch new"
 * add "A" back into the message store
 * re-run "notmuch new"

Then notmuch sees the messages as distinct threads.

This happens because if you insert "B" initially (before anything is
known about "A"), then a "ghost message" gets added to the database in
reference to "A" that is in the same thread, which "A" takes over when
it appears.

But if "A" is subsequently removed, no ghost message is retained, so
when "A" appears, it is treated as a new thread.

I see a few options to fix this:

ghost-on-removal
----------------

We could unilaterally add a ghost upon message removal.  This has a
few disadvantages: the message index would leak information about what
messages the user has ever been exposed to, and we also create a
perpetually-growing dataset -- the ghosts can never be removed.

ghost-on-removal-when-shared-thread-exists
------------------------------------------

We could add a ghost upon message removal iff there are other
non-ghost messages with the same thread ID.

We'd also need to remove all ghost messages that share a thread when
the last non-ghost message in that thread is removed.

This still has a bit of information leakage, though: the message index
would reveal that i've seen a newer message in a thread, even if i had
deleted it from my message store

track-dependencies
------------------

rather than a simple "ghost-message" we could store all the (A,B)
message-reference pairs internally, showing which messages A reference
which other messages B.

Then removal of message X would require deleting all message-reference
pairs (X,B), and only deleting a ghost message if no (A,X) reference
pair exists.

This requires modifying the database by adding a new and fairly weird
table that would need to be indexed by both columns.  I don't know
whether xapian has nice ways to do that.

scan-dependencies
-----------------

Without modifying the database, we could do something less efficient.

Upon removal of message X, we could scan the headers of all non-ghost
messages that share a thread with X.  If any of those messages refers
to X, we would add a ghost message.  If none of them do, then we would
just drop X entirely from the table.

---------------------

One risk of attempted fixes to this problem is that we could fail to
remove the search term indexes entirely.  This test contains
additional subtests to guard against that.

This test also ensures that the right number of ghost messages exist
in each situation; this will help us ensure we don't accumulate ghosts
indefinitely or leak too much information about what messages we've
seen or not seen, while still making it easy to reassemble threads
when messages come in out-of-order.

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