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

src/wp-admin/includes/post.php

@@ -1541,6 +1541,15 @@ function wp_create_post_autosave( $post_data ) {
 			return 0;
 		}
 
+		/**
+		 * Fires before an autosave is stored.
+		 *
+		 * @since 4.1.0
+		 *
+		 * @param array $new_autosave Post array - the autosave that is about to be saved.
+		 */
+		do_action( 'wp_creating_autosave', $new_autosave );
+
 		return wp_update_post( $new_autosave );
 	}