Post format should be revisioned, auto-saved and previewable

[kovshenin]

Like the post format metadata in #23539, the post format itself should be revisioned. Steps to reproduce:

1. Publish a standard post
2. Change the format to image, set the content to "this is an image", update
3. Change the format to quote, set the content to "this is a quote", update
4. Restore the "this is an image" revision

The post is restored to the "this is an image" post, but the format is still a quote. This could be part of a bigger "revisions for taxonomies" feature.

[kovshenin]

23893.diff​ is an approach similar to [23859] and [23862].

[kovshenin]

It looks like the change in supports on the revision post type is not needed. I think I left it there because I was using get_post_format on revision posts when debugging.

[kovshenin]

23893.2.diff​ uses post meta to store revisioned terms for post formats and introduces a more generic approach when handling autosaves. Categories and tags could easily fit into this.

[kovshenin]

On second thought, I think doing terms revision is overkill for now and would rather go with storing the format in post meta. A full-blown tax/terms revision framework can go in after #20564 is closed.

[kovshenin]

There's ​a patch for this sitting in #20564.

[kovshenin]

Fixed in [23928]