Do not load special pages in WatchedItem::load

WatchedItem::load was looking for a special page in the database:
SELECT  wl_notificationtimestamp 
  FROM `watchlist`   
 WHERE wl_user = '1' 
   AND wl_namespace = '-1' 
   AND wl_title = 'Log' 
 LIMIT 1  
But it is known that such a page never will stay in the database,
so added a check to avoid the select.

Change-Id: I8c65328a32aa93c351ac476ce79d12d2835128f6
Merge "Do not load special pages in WatchedItem::load"