Workaround for T136644

Converting AbuseFilter to extension registration caused config
lines like $x = array_diff($x, ...) to break as they run before
the extension globals are registered. As a quick hack, copy the
relevant variable; will be fixed later by changing it into an
associative array.

Bug: T136644
Change-Id: I6332d84ecdc72ae8cabb787f4a10e6b59e7e4c2a