Filters the list of fields saved in post revisions.

Included by default: ‘post_title’, ‘post_content’ and ‘post_excerpt’.

Disallowed fields: ‘ID’, ‘post_name’, ‘post_parent’, ‘post_date’,
‘post_date_gmt’, ‘post_status’, ‘post_type’, ‘comment_count’,
and ‘post_author’.

$fields = apply_filters( '_wp_post_revision_fields', $fields, $post );