[Fix] closing of collapsible directories in NERDTree plugin

[juliuszint]

The NERDTree plugin will under some conditions fail to close a collapisble directory with the error message: "cannot close tree root". This commit replaces the traversing logic for collapsible directories when closing with the latest code from NERDTree.

To reproduce this error use g:NERDTreeCascadeSingleChildDir=0 and g:NERDTreeCascadeOpenSingleChildDir=1. Then the following directory tree:

dir-a
    dir-b
        dir-c
dir-d

Go over dir-c and press "x" to close this node.

Requirements (please check off with 'x')

• [x] I have read the Contributing Guidelines
• [x] I have read or at least glanced at the FAQ
• [x] I have read or at least glanced at the Wiki