Index: docs/hooks.txt
===================================================================
--- docs/hooks.txt	(revision 39564)
+++ docs/hooks.txt	(working copy)
@@ -922,6 +922,11 @@
 $pager: the pager
 $queryInfo: the query parameters
 
+'PageHistoryTool' : at the end of the tools line to allow extra tools to be added
+$this: PageHistory object
+$rev: page revision
+&$tools: array of tool links to be constructed
+
 'PageRenderingHash': alter the parser cache option hash key
   A parser extension which depends on user options should install
   this hook and append its values to the key.
Index: includes/PageHistory.php
===================================================================
--- includes/PageHistory.php	(revision 39564)
+++ includes/PageHistory.php	(working copy)
@@ -326,6 +326,8 @@
 				);
 				$tools[] = "<span class=\"mw-history-undo\">{$undolink}</span>";
 			}
+			#Allow extensions with tools such as Edit Summary to add their links
+			wfRunHooks( 'PageHistoryTool', array( $this, $rev, &$tools) );
 		}
 
 		if( $tools ) {