This is useful for speeding up indexing runs when only Xapian
rules change but SQLite indexing doesn't change. This mostly
implies `--reindex', but does NOT pick up new messages (because
SQLite indexing needs to occur for that).
I'm leaving this undocumented in the manpage for now since it's
mainly to speed up development and testing. Users upgrading to
1.6.0 will be advised to `--reindex --rethread', anyways, due to
the threading improvements since 1.1.0-pre1.
It may make sense to document for 1.7+ when there's Xapian-only
indexing changes, though.