Post locks:
- Show 'Saving revision...' while autosaving after a post has been taken over. Change to 'Your latest changes were saved as a revision.' when autosave completes.
- Make sure a user exists before using $user->display_name.
- Add 'post_lock_text' action for extending the message text.

git-svn-id: https://core.svn.wordpress.org/trunk@24042 1a063a9b-81f0-0310-95a4-ce76da25c4cd