bugfix (Storing several pages in a loop creates only one page)

When several pages are created during one MW run (e.g. using runJobs.php), $target_title->exists() does not work correctly.
It does not use Title::GAID_FOR_UPDATE, which is needed to get correct data from cache for newly created pages.
Replaced by a call to $target_title->getArticleID().

Change-Id: I73b9e9f2cd4aeed73f28737fdba15639b40258d7