Introduce some actions and filters which aid plugins in revisioning post meta.

 * `wp_save_post_revision_post_has_changed` filter which can be used to determine if a post has been changed, and therefore if a revision should be created for a post.
 * `wp_get_revision_ui_diff` filter which can be used to filter the fields displayed in the post revision diff UI.
 * `wp_creating_autosave` action which is fired just before an autosave is created.

See #20564.
Props mattheu, adamsilverstein.


git-svn-id: https://develop.svn.wordpress.org/trunk@30091 602fd350-edb4-49c9-b593-d223f7449a82