Silence possible warnings from is_dir(). fixes #25572.

Built from https://develop.svn.wordpress.org/trunk@25764


git-svn-id: http://core.svn.wordpress.org/trunk@25677 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -1507,7 +1507,7 @@ class WP_Automatic_Upgrader {
 		// Search all directories we've found for evidence of version control.
 		foreach ( $vcs_dirs as $vcs_dir ) {
 			foreach ( $check_dirs as $check_dir ) {
-				if ( $checkout = is_dir( rtrim( $check_dir, '\\/' ) . "/$vcs_dir" ) )
+				if ( $checkout = @is_dir( rtrim( $check_dir, '\\/' ) . "/$vcs_dir" ) )
 					break 2;
 			}
 		}