Improve _fix_attachment_links(), replace attachment URLs with the real...

Improve _fix_attachment_links(), replace attachment URLs with the real permalink only for published posts, re-save only when there are changes, see #13429

git-svn-id: https://core.svn.wordpress.org/trunk@20308 1a063a9b-81f0-0310-95a4-ce76da25c4cd