Set $parent_file for post-new.php when show_in_menu is a file. Provides for...

Set $parent_file for post-new.php when show_in_menu is a file. Provides for correct top-level highlighting. $submenu_file remains set to expect a post-new.php menu. see #19125.

git-svn-id: https://core.svn.wordpress.org/trunk@19374 1a063a9b-81f0-0310-95a4-ce76da25c4cd