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_file_node.vim

@@ -83,7 +83,7 @@ endfunction
 "Return:
 "a string that can be used in the view to represent this node
 function! s:TreeFileNode.displayString()
-    return self.path.displayString()
+    return self.path.flagSet.renderToString() . self.path.displayString()
 endfunction
 
 "FUNCTION: TreeFileNode.equals(treenode) {{{1