MDL-56082 auth: minor tweak to login block idp links

No longer set the return url to $SESSION->wantsurl, it doens't make
sense in this context, as the user is probably on the page they want.

Also add a note about this functionality to auth/ugrade.txt

auth/upgrade.txt

@@ -5,6 +5,7 @@ information provided here is intended especially for developers.
 
 * New auth hook - pre_user_login_hook() - available, triggered right after the user object is created.
   This can be used to modify the user object before any authentication errors are raised.
+* The block_login now displays the loginpage_idp_list() links as well as main login page.
 
 === 3.0 ===
 

blocks/login/block_login.php

@@ -106,7 +106,7 @@ function get_content () {
             $potentialidps = array();
             foreach ($authsequence as $authname) {
                 $authplugin = get_auth_plugin($authname);
-                $potentialidps = array_merge($potentialidps, $authplugin->loginpage_idp_list(empty($SESSION->wantsurl) ? '' : $SESSION->wantsurl));
+                $potentialidps = array_merge($potentialidps, $authplugin->loginpage_idp_list($this->page->url->out(false)));
             }
 
             if (!empty($potentialidps)) {