Revisions: add the $post parameter to _wp_post_revision_fields

[jeherve]

An additional $post parameter was added to the filter in core in 4.5: https://core.trac.wordpress.org/changeset/36659

Adding it to the plugin avoids issues when other plugins make use of the filter.

Reported here: https://wordpress.org/support/topic/show-warning-on-jetpack-plugin/