fix flag rendering for cascading dirs

Use the innermost dirs flags only. Previously each dirs flags were
rendered inline i.e.

    [x]dir1/[y]dir2/[z]dir3/

This looked bad and broke the UI. Now we only render the flags for dir3
- and render them on the left side of the cascade.

Not sure if this is ultimately the right decision - but will do for now.

lib/nerdtree/tree_dir_node.vim

@@ -84,7 +84,9 @@ function! s:TreeDirNode.displayString()
 
     let sym = cascade[-1].isOpen ? g:NERDTreeDirArrowCollapsible : g:NERDTreeDirArrowExpandable
 
-    return sym . ' ' . rv
+    let flags = cascade[-1].path.flagSet.renderToString()
+
+    return sym . ' ' . flags . rv
 endfunction
 
 "FUNCTION: TreeDirNode.findNode(path) {{{1