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.

Built from https://develop.svn.wordpress.org/trunk@30091


git-svn-id: http://core.svn.wordpress.org/trunk@30091 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Comment Template Tags: Improve the availability of context information in comment template tag filters.

Fixes #22380 props momo360modena, tareq1988.

Built from https://develop.svn.wordpress.org/trunk@30092


git-svn-id: http://core.svn.wordpress.org/trunk@30092 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Support an empty string passed as a status in `WP_Comment_Query`.

The changes in [30084] broke backward compatibility with interfaces that
manually passed an empty string for the value of 'status', such as on
wp-admin/edit-comments.php.

Fixes #29612.
Built from https://develop.svn.wordpress.org/trunk@30093


git-svn-id: http://core.svn.wordpress.org/trunk@30093 1a063a9b-81f0-0310-95a4-ce76da25c4cd