Change bg color of collapsed diff to blue on hover

app/assets/stylesheets/framework/blocks.scss

@@ -16,9 +16,14 @@
   font-weight: normal;
   font-size: 16px;
   line-height: 36px;
+
   &.diff-collapsed {
     padding: 5px;
     cursor: pointer;
+
+    &:hover {
+      background-color: $row-hover;
+    }
   }
 }