adds: @DIFFPAGE@ placeholder to templates

TODO:
use rev1, rev2
http://wintersunset.delfi.lan/dokuwiki/googledrawing?do=diff&rev1=1345291381&rev2=&difftype=sidebyside

--- dokuwiki-2014-05-06/inc/html.php	2014-05-08 11:27:02.548232655 +0300
+++ dokuwiki-2014-05-06/inc/html.php	2014-05-08 11:27:02.548232655 +0300
@@ -1134,7 +1134,13 @@
     $rev1 = $REV;
 
     $rev2 = $INPUT->ref('rev2');
-    if(is_array($rev2)) {
+
+    // handle $r1 + $r2, see DW #1821
+    if ($INPUT->has('r1') && $INPUT->has('r2')) {
+        $rev1 = $INPUT->int('r1');
+        $rev2 = $INPUT->int('r2');
+
+    } elseif(is_array($rev2)){
         $rev1 = (int) $rev2[0];
         $rev2 = (int) $rev2[1];
 
@@ -1458,8 +1464,8 @@
     } else {
         $urlparam = array(
             'do' => 'diff',
-            'rev2[0]' => $lrev,
-            'rev2[1]' => $rrev,
+            'r1' => $lrev,
+            'r2' => $rrev,
             'difftype' => $difftype,
         );
     }
--- dokuwiki-2020-07-29a/inc/Subscriptions/PageSubscriptionSender.php~	2023-03-27 11:45:18.000000000 +0300
+++ dokuwiki-2020-07-29a/inc/Subscriptions/PageSubscriptionSender.php	2023-03-27 11:45:20.809303977 +0300
@@ -39,6 +39,7 @@
         if ($rev) {
             $subject = 'changed';
             $trep['OLDPAGE'] = wl($id, "rev=$rev", true, '&');
+            $trep['DIFFPAGE']= wl($id,"do=diff&r1=$rev&r2=$current_rev",true,'&');
 
             $old_content = rawWiki($id, $rev);
             $new_content = rawWiki($id);
@@ -62,6 +63,7 @@
         } else {
             $subject = 'newpage';
             $trep['OLDPAGE'] = '---';
+            $trep['DIFFPAGE']= '---';
             $tdiff = rawWiki($id);
             $hdiff = nl2br(hsc($tdiff));
         }