Fix sticky diff header background (#27697)

Fixes: https://github.com/go-gitea/gitea/issues/27604

Add negative margins so the header covers any shadow of active elements.
No rendering change of the content of the header because the padding
counteracts the effect.

<img width="128" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/3d0f55b6-9351-4985-a290-da9a92d15b4e">

web_src/css/repo.css

@@ -1497,7 +1497,8 @@
   position: sticky;
   top: 0;
   z-index: 8;
-  padding: 7px 0;
+  padding: 7px 5px;
+  margin: 0 -5px; /* negative margin so it covers active file shadow */
   height: 44px; /* this height should match sticky-2nd-row */
   background: var(--color-body);
 }