Changed to calls to Article::getContent() to Article::getRawText():

Changed to calls to Article::getContent() to Article::getRawText():
- in Article::insertRedirect(): if this will be saved in the redirect table, it should only be checked with the current text (in fact this is never called when viewing an old revision, but just in case someone breaks this)
- in Article::prepareTextForEdit(): as for Article::doEdit() this should be actual page's contents