Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 8462)
+++ wp-includes/pluggable.php	(working copy)
@@ -1514,7 +1514,8 @@
 
 	// PEAR Text_Diff is lame; it includes things from include_path rather than it's own path.
 	// Not sure of the ramifications of disttributing modified code.
-	ini_set('include_path', '.' . PATH_SEPARATOR . ABSPATH . WPINC );
+	if ( ! @set_include_path( @get_include_path() . PATH_SEPARATOR . ABSPATH . WPINC ) )
+		return __('Error: the PHP function set_include_path() is disabled on your server. Post revisions cannot be displayied. Please contact your hosting company to resolve this error.');
 
 	if ( !class_exists( 'WP_Text_Diff_Renderer_Table' ) )
 		require( ABSPATH . WPINC . '/wp-diff.php' );
@@ -1534,8 +1535,6 @@
 	$renderer  = new WP_Text_Diff_Renderer_Table();
 	$diff = $renderer->render($text_diff);
 
-	ini_restore('include_path');
-
 	if ( !$diff )
 		return '';