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.
Merges [42214] to the 4.9 branch.
Fixes #42628 for 4.9.

Built from https://develop.svn.wordpress.org/branches/4.9@42215


git-svn-id: http://core.svn.wordpress.org/branches/4.9@42044 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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;
 			}
 		}
 

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.1-alpha-42213';
+$wp_version = '4.9.1-alpha-42215';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.