When saving Custom CSS, preserve the original version in the revision history.

[georgestephanis]

Related: #30854

When saving CSS revisions via wp_update_custom_css_post() the initial css is lost, and only subsequent updates are versioned in revisions in the database.

To avoid data loss, @adamsilverstein demonstrated that calling wp_update_post( $post ) immediately after insertion can create the revision correctly as a stopgap measure here: https://core.trac.wordpress.org/attachment/ticket/30854/test_wp_first_revision_is_not_lost.2.diff

This is only a targeted fix, and will not preserve the first revision of posts managed via the new WP REST API endpoints.

[westonruter]

The scope of the patch makes sense for 4.7, except we should use wp_save_post_revision().

[westonruter]

In 39477:

Customize: Ensure a custom_css post insertion gets an initial post revision.

Props georgestephanis, westonruter.
See #30854, #38672, #35395.
Fixes #39032.

[westonruter]

In 39479:

Customize: Ensure a custom_css post insertion gets an initial post revision.

Merges [39477] onto 4.7 branch.
Props georgestephanis, westonruter.
Fixes #39032 for 4.7.