Fixed positioning of the diff tree list on mobile

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59739

app/assets/stylesheets/pages/diff.scss

@@ -1059,7 +1059,10 @@ table.code {
     flex-direction: column;
     .diff-tree-list {
-      width: 100%;
+      position: relative;
+      top: 0;
+      // !important is required to override inline styles of resizable sidebar
+      width: 100% !important;
     }
     .tree-list-holder {