Index: wp-admin/admin.php
===================================================================
--- wp-admin/admin.php	(revision 18542)
+++ wp-admin/admin.php	(working copy)
@@ -31,33 +31,29 @@
 
 if ( get_option('db_upgraded') ) {
 	$wp_rewrite->flush_rules();
-	update_option( 'db_upgraded',  false );
+	update_option( 'db_upgraded', false );
 
-	/**
-	 * Runs on the next page load after successful upgrade
-	 *
-	 * @since 2.8
-	 */
-	do_action('after_db_upgrade');
-} elseif ( get_option('db_version') != $wp_db_version ) {
-	if ( !is_multisite() ) {
-		wp_redirect(admin_url('upgrade.php?_wp_http_referer=' . urlencode(stripslashes($_SERVER['REQUEST_URI']))));
+	// Runs on the next page load after successful upgrade
+	do_action( 'after_db_upgrade' );
+
+} elseif ( get_option( 'db_version' ) != $wp_db_version ) {
+	if ( ! is_multisite() ) {
+		define( 'WP_INSTALLING', true );
+		require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
+		delete_site_transient( 'update_core' );
+		wp_upgrade();
+		wp_redirect( $_SERVER['REQUEST_URI'] );
 		exit;
 	} elseif ( apply_filters( 'do_mu_upgrade', true ) ) {
-		/**
-		 * On really small MU installs run the upgrader every time,
-		 * else run it less often to reduce load.
-		 *
-		 * @since 2.8.4b
-		 */
+		// On really small MU installs run the upgrader every time,
+		// else run it less often to reduce load.
 		$c = get_blog_count();
-		if ( $c <= 50 || ( $c > 50 && mt_rand( 0, (int)( $c / 50 ) ) == 1 ) ) {
+		if ( $c <= 50 || ( $c > 50 && mt_rand( 0, (int) ( $c / 50 ) ) == 1 ) ) {
 			require_once( ABSPATH . WPINC . '/http.php' );
-			$response = wp_remote_get( admin_url( 'upgrade.php?step=1' ), array( 'timeout' => 120, 'httpversion' => '1.1' ) );
-			do_action( 'after_mu_upgrade', $response );
-			unset($response);
+			wp_upgrade();
+			do_action( 'after_mu_upgrade', '' );
 		}
-		unset($c);
+		unset( $c );
 	}
 }
 
Index: wp-admin/upgrade.php
===================================================================
--- wp-admin/upgrade.php	(revision 18542)
+++ wp-admin/upgrade.php	(working copy)
@@ -17,94 +17,12 @@
 /** Load WordPress Bootstrap */
 require( '../wp-load.php' );
 
-timer_start();
 require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
+delete_site_transient( 'update_core' );
 
-delete_site_transient('update_core');
-
-if ( isset( $_GET['step'] ) )
-	$step = $_GET['step'];
-else
-	$step = 0;
-
-// Do it.  No output.
-if ( 'upgrade_db' === $step ) {
-	wp_upgrade();
+wp_upgrade();
+if ( 'upgrade_db' === $_GET['step'] )
 	die( '0' );
-}
 
-$step = (int) $step;
-
-$php_version    = phpversion();
-$mysql_version  = $wpdb->db_version();
-$php_compat     = version_compare( $php_version, $required_php_version, '>=' );
-$mysql_compat   = version_compare( $mysql_version, $required_mysql_version, '>=' ) || file_exists( WP_CONTENT_DIR . '/db.php' );
-
-@header( 'Content-Type: ' . get_option( 'html_type' ) . '; charset=' . get_option( 'blog_charset' ) );
-?>
-<!DOCTYPE html>
-<html xmlns="http://www.w3.org/1999/xhtml" <?php language_attributes(); ?>>
-<head>
-	<meta http-equiv="Content-Type" content="<?php bloginfo( 'html_type' ); ?>; charset=<?php echo get_option( 'blog_charset' ); ?>" />
-	<title><?php _e( 'WordPress &rsaquo; Update' ); ?></title>
-	<?php
-	wp_admin_css( 'install', true );
-	wp_admin_css( 'ie', true );
-	?>
-</head>
-<body>
-<h1 id="logo"><img alt="WordPress" src="images/wordpress-logo.png" /></h1>
-
-<?php if ( get_option( 'db_version' ) == $wp_db_version || !is_blog_installed() ) : ?>
-
-<h2><?php _e( 'No Update Required' ); ?></h2>
-<p><?php _e( 'Your WordPress database is already up-to-date!' ); ?></p>
-<p class="step"><a class="button" href="<?php echo get_option( 'home' ); ?>/"><?php _e( 'Continue' ); ?></a></p>
-
-<?php elseif ( !$php_compat || !$mysql_compat ) :
-	if ( !$mysql_compat && !$php_compat )
-		printf( __('You cannot update because <a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> requires PHP version %2$s or higher and MySQL version %3$s or higher. You are running PHP version %4$s and MySQL version %5$s.'), $wp_version, $required_php_version, $required_mysql_version, $php_version, $mysql_version );
-	elseif ( !$php_compat )
-		printf( __('You cannot update because <a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> requires PHP version %2$s or higher. You are running version %3$s.'), $wp_version, $required_php_version, $php_version );
-	elseif ( !$mysql_compat )
-		printf( __('You cannot update because <a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> requires MySQL version %2$s or higher. You are running version %3$s.'), $wp_version, $required_mysql_version, $mysql_version );
-?>
-<?php else :
-switch ( $step ) :
-	case 0:
-		$goback = stripslashes( wp_get_referer() );
-		$goback = esc_url_raw( $goback );
-		$goback = urlencode( $goback );
-?>
-<h2><?php _e( 'Database Update Required' ); ?></h2>
-<p><?php _e( 'WordPress has been updated! Before we send you on your way, we have to update your database to the newest version.' ); ?></p>
-<p><?php _e( 'The update process may take a little while, so please be patient.' ); ?></p>
-<p class="step"><a class="button" href="upgrade.php?step=1&amp;backto=<?php echo $goback; ?>"><?php _e( 'Update WordPress Database' ); ?></a></p>
-<?php
-		break;
-	case 1:
-		wp_upgrade();
-
-			$backto = !empty($_GET['backto']) ? stripslashes( urldecode( $_GET['backto'] ) ) :  __get_option( 'home' ) . '/';
-			$backto = esc_url( $backto );
-			$backto = wp_validate_redirect($backto, __get_option( 'home' ) . '/');
-?>
-<h2><?php _e( 'Update Complete' ); ?></h2>
-	<p><?php _e( 'Your WordPress database has been successfully updated!' ); ?></p>
-	<p class="step"><a class="button" href="<?php echo $backto; ?>"><?php _e( 'Continue' ); ?></a></p>
-
-<!--
-<pre>
-<?php printf( __( '%s queries' ), $wpdb->num_queries ); ?>
-
-<?php printf( __( '%s seconds' ), timer_stop( 0 ) ); ?>
-</pre>
--->
-
-<?php
-		break;
-endswitch;
-endif;
-?>
-</body>
-</html>
+wp_safe_redirect( wp_get_referer() );
+exit;
\ No newline at end of file
