Prevent duplicate image loading (go-gitea#25675) (go-gitea#25684)

Backport go-gitea#25675 by @delvh

Regression of go-gitea#25672.

Co-authored-by: delvh <dev.lh@web.de>

web_src/js/features/imagediff.js

@@ -65,8 +65,9 @@ export function initImageDiff() {
     };
   }
 
-  $('.image-diff').each(function() {
+  $('.image-diff:not([data-image-diff-loaded])').each(function() {
     const $container = $(this);
+    $container.attr('data-image-diff-loaded', 'true');
 
     // the container may be hidden by "viewed" checkbox, so use the parent's width for reference
     const diffContainerWidth = Math.max($container.closest('.diff-file-box').width() - 300, 100);