Merge branch 'mr-file-tree-fixed-safari' into 'master'

Fixed merge request file tree not sticking on Safari

Closes #52277

See merge request gitlab-org/gitlab-ce!22147

app/assets/stylesheets/pages/diff.scss

@@ -1000,6 +1000,7 @@
 }
 
 .tree-list-holder {
+  position: -webkit-sticky;
   position: sticky;
   top: 100px;
   max-height: calc(100vh - 100px);