Convert image diff background image to CSS

CHANGELOG

@@ -29,6 +29,7 @@ v 8.11.0 (unreleased)
   - Bump gitlab_git to speedup DiffCollection iterations
   - Make branches sortable without push permission !5462 (winniehell)
   - Check for Ci::Build artifacts at database level on pipeline partial
+  - Convert image diff background image to CSS (ClemMakesApps)
   - Make "New issue" button in Issue page less obtrusive !5457 (winniehell)
   - Gitlab::Metrics.current_transaction needs to be public for RailsQueueDuration
   - Add GitLab Workhorse version to admin dashboard (Katarzyna Kobierska Ula Budziszewska)

app/assets/stylesheets/pages/diff.scss

@@ -164,7 +164,10 @@
       line-height: 0;
       img {
         border: 1px solid #fff;
-        background: image-url('trans_bg.gif');
+        background-image: linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%),
+        linear-gradient(45deg, #e5e5e5 25%, transparent 25%, transparent 75%, #e5e5e5 75%, #e5e5e5 100%);
+        background-size: 10px 10px;
+        background-position: 0 0, 5px 5px;
         max-width: 100%;
       }
       &.deleted {