Restore unification of multi-page post content in Reader mode (#4547)

* Restore unification of multi-page post content in Reader mode

* Combine comments

Co-Authored-By: Alain Schlesser <alain.schlesser@gmail.com>

* Fix Squiz.WhiteSpace.SuperfluousWhitespace.EndLine

Co-authored-by: Alain Schlesser <alain.schlesser@gmail.com>

includes/templates/class-amp-post-template.php

@@ -342,8 +342,17 @@ private function build_post_comments_data() {
 	 * Build post content.
 	 */
 	private function build_post_content() {
-		/** This filter is documented in wp-includes/post-template.php */
-		$content = apply_filters( 'the_content', get_the_content( null, false, $this->post ) );
+		if ( post_password_required( $this->post ) ) {
+			$content = get_the_password_form( $this->post );
+		} else {
+			/**
+			 * This filter is documented in wp-includes/post-template.php.
+			 *
+			 * Note: This is intentionally not using get_the_content() because the legacy behavior of posts in
+			 * Reader mode is to display multi-page posts as a single page without any pagination links.
+			 */
+			$content = apply_filters( 'the_content', $this->post->post_content );
+		}
 
 		$this->add_data_by_key( 'post_amp_content', $content );
 	}