Restoring ThomasV's patch for a hook & use of the existing $cache parameter on addPri...

Restoring ThomasV's patch for a hook & use of the existing $cache parameter on addPrimaryWikiText.
Further testing indicates the problem was probably an incomplete sync or bad caching of
OutputPage.php on some servers, so the broken version of the function was called
on old page views, incorrectly storing old text items into the parser cache.