Properly peform deletion updates when moving over a redirect

When moving over a redirect, WikiPage::doDeleteUpdates is called to do
things like clean up obsolete entries in `redirect` and other tables.
But it doesn't pass a Content object and the WikiPage doesn't have one
cached, so it winds up not actually doing these updates.

Change-Id: I54c90cebfd6bea07137a77bd70acec02cab51084