Fix default value of the SpecialWatchlistFilters hook

The documented default value of the SpecialWatchlistFilters hook
didn't work. Fixed this to work like the built in defaults.

This keeps the b/c with uses which default to false (the only
use I'm aware of is in FlaggedRevs and that defaults to false)

(I want to use this in Wikibase Client)

Change-Id: I79811e9f3e7431e3403cf93674667d624d3d4db2
Merge "Fix default value of the SpecialWatchlistFilters hook"