extindex: support --rethread and content bifurcation

--rethread is useful for dealing with bugs and behaves
just like it does with current inboxes.

This is in case our content deduplication logic changes for
whatever reason and causes previously merged messages to be
considered "different".  As with v2, this won't allow us to
merge messages in a way that allows deduplicating messages which
were previously considered different, but v2 inboxes do not
allow that, either.

In other words, this makes the --reindex and --rethread
switches of -extindex match the behavior of v2 -index.

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