Themes: Ensure the theme roots cache is cleared when registering a theme directory.

Props soulseekah, johnbillion

Fixes #43228

Built from https://develop.svn.wordpress.org/trunk@42788


git-svn-id: http://core.svn.wordpress.org/trunk@42618 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -413,6 +413,8 @@ function register_theme_directory( $directory ) {
 		$wp_theme_directories[] = $untrailed;
 	}
 
+	wp_clean_themes_cache();
+
 	return true;
 }
 

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42787';
+$wp_version = '5.0-alpha-42788';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.