Merging topics hangs when users are subscribed

[Studio384]

If someone is subscribed to a topic that's being merged, the process grinds to a halt because $db->result returns the first row repeatedly. I can confirm that this breaks with both Postgres and SQLite. I'm guessing behavior is different with the MySQL backend, or else this probably would have been noticed and fixed by this point.

http://fluxbb.org/development/core/tickets/905/#qa0e59z22