Use a better function to change directories

When g:NERDTreeChDirMode is 2, changing the tree root will change the working
directory as well.  This change was silent because the wrong function was used to
make the switch.  This commit uses a better function that echoes a message.

lib/nerdtree/nerdtree.vim

@@ -21,7 +21,7 @@ function! s:NERDTree.changeRoot(node)
 
     "change dir to the dir of the new root if instructed to
     if g:NERDTreeChDirMode ==# 2
-        exec "cd " . self.root.path.str({'format': 'Edit'})
+        call self.root.path.changeToDir()
     endif
 
     call self.render()