REST API: Support nested _fields in revisions controller

[ellatrix]

WP_REST_Revisions_Controller::prepare_item_for_response() uses in_array() for content, title, excerpt, and guid fields, which doesn't support nested _fields syntax (e.g. _fields=content.raw).

PR: ​https://github.com/WordPress/wordpress-develop/pull/11230

https://core.trac.wordpress.org/ticket/64844

Use rest_is_field_included() instead of in_array() for content, title, excerpt, and guid fields in WP_REST_Revisions_Controller. This lets clients request specific sub-fields (e.g. _fields=content.raw) and skip expensive the_content rendering.

The ​REST API `_fields` documentation states that nested fields are supported using dot notation. However, the revisions controller uses in_array() which doesn't match nested keys like content.raw.

Several other controllers already use rest_is_field_included() for raw/rendered sub-fields:

• ​`WP_REST_Posts_Controller` — title.raw, title.rendered, content.raw, content.rendered
• ​`WP_REST_Templates_Controller` — content.raw, title.raw, title.rendered
• ​`WP_REST_Menu_Items_Controller` — title.raw, title.rendered
• ​`WP_REST_Global_Styles_Controller` — title.raw, title.rendered
• ​`WP_REST_Themes_Controller` — generic {field}.raw, {field}.rendered

Related Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/76347

Thanks for the review! I'm going to leave it as-is since we're close to Beta 5.

[ellatrix]

In 61987:

REST API: Support nested _fields in revisions controller.

Use rest_is_field_included() instead of in_array() for content, title, excerpt, and guid fields in WP_REST_Revisions_Controller. This lets clients request specific sub-fields (e.g. _fields=content.raw) and skip expensive the_content rendering.

The [REST API _fields documentation](​https://developer.wordpress.org/rest-api/using-the-rest-api/global-parameters/#_fields) states that nested fields are supported using dot notation. However, the revisions controller uses in_array() which doesn't match nested keys like content.raw.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/11230.

Props ellatrix, andrewserong, mukeshpanchal27.
Fixes #64844.