Fix download archive menu for tree view.

This fixes the issue of the menu going outside of the main `container`.

app/views/projects/tree/show.html.haml

 %div.tree-ref-holder
   = render 'shared/ref_switcher', destination: 'tree', path: @path
 - if can? current_user, :download_code, @project
-  .tree-ref-holder.pull-right
-    = render 'projects/repositories/download_archive', ref: @ref, btn_class: 'btn-group-small', split_button: true
+  = render 'projects/repositories/download_archive', ref: @ref, btn_class: 'btn-group-small tree-ref-holder pull-right', split_button: true
 %div#tree-holder.tree-holder
   = render "tree", tree: @tree