Theme Upgrader: Set is_multi to true when doing bulk upgrades.

fixes #29234.

git-svn-id: https://develop.svn.wordpress.org/trunk@29622 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-upgrader.php

@@ -1045,6 +1045,7 @@ class Theme_Upgrader extends WP_Upgrader {
 				'destination' => get_theme_root( $theme ),
 				'clear_destination' => true,
 				'clear_working' => true,
+				'is_multi' => true,
 				'hook_extra' => array(
 					'theme' => $theme
 				),