Finalize the code which redirects to about.php after a core update. Only...

Finalize the code which redirects to about.php after a core update. Only triggers on update-core.php in case wp_update_core() is called in a different context.
props ocean90 for initial patch. fixes #18467.


git-svn-id: https://core.svn.wordpress.org/trunk@19524 1a063a9b-81f0-0310-95a4-ce76da25c4cd