Always force <title> to 'Edit Post' (or equivalent)

It needs to be in a more generic state. Setting the global `$title`
overrides the behavior of `get_admin_page_title()` while still letting
the value be filterable via `admin_title`.

gutenberg.php

@@ -175,6 +175,8 @@ function gutenberg_pre_init() {
  * @return bool   Whether Gutenberg was initialized.
  */
 function gutenberg_init( $return, $post ) {
+	global $title, $post_type;
+
 	if ( true === $return && current_filter() === 'replace_editor' ) {
 		return $return;
 	}
@@ -187,6 +189,16 @@ function gutenberg_init( $return, $post ) {
 	add_filter( 'screen_options_show_screen', '__return_false' );
 	add_filter( 'admin_body_class', 'gutenberg_add_admin_body_class' );
 
+	$post_type_object = get_post_type_object( $post_type );
+	/**
+	 * Always force <title> to 'Edit Post' (or equivalent)
+	 * because it needs to be in a generic state for both
+	 * post-new.php and post.php?post=<id>.
+	 */
+	if ( ! empty( $post_type_object ) ) {
+		$title = $post_type_object->labels->edit_item;
+	}
+
 	/**
 	 * Remove the emoji script as it is incompatible with both React and any
 	 * contenteditable fields.