Revisions:
- Store the post format as meta on revisions (including autosaves).
- Add post formats data (post meta) when autosaving.
- Only add non-empty post formats data to revisions.
- Correct the post format when previewing a published post.
Props kovshenin, see #19570, see #20564.


git-svn-id: https://core.svn.wordpress.org/trunk@23928 1a063a9b-81f0-0310-95a4-ce76da25c4cd