Avoid refetching a post in get_post() when passed a post object that has...

Avoid refetching a post in get_post() when passed a post object that has already been raw sanitized.  Clean up sanitize loop.  Props scribu. fixes #18822

git-svn-id: https://core.svn.wordpress.org/trunk@18846 1a063a9b-81f0-0310-95a4-ce76da25c4cd