Upgrade: Fix updating plugins which include a numeric file/folder names.

The fix in [41821] caused numeric folder names to be reindexed to 0..n when in the root directory (for example, `my-plugin/24/`).

Props edo888.
See #41524.
Fixes #42628 for trunk.


git-svn-id: https://develop.svn.wordpress.org/trunk@42214 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -342,7 +342,8 @@ class WP_Upgrader {
 			if ( ! empty( $details['files'] ) ) {
 				$children = $this->flatten_dirlist( $details['files'], $path . $name . '/' );
 
-				$files = array_merge( $files, $children );
+				// Merge keeping possible numeric keys, which array_merge() will reindex from 0..n
+				$files = $files + $children;
 			}
 		}