Clean up the handler for the "x" mapping (#767)

Previously, pressing "x" on the tree root would result in
unpredictable behavior.  The user would either an receive an error
message or the parent of the tree root (which is not visible) would
be closed.  This commit repairs this problem.

In addition, some code duplication was removed.

autoload/nerdtree/ui_glue.vim

@@ -184,24 +184,28 @@ function! s:closeChildren(node)
 endfunction
 
 " FUNCTION: s:closeCurrentDir(node) {{{1
-" closes the parent dir of the current node
+" Close the parent directory of the current node.
 function! s:closeCurrentDir(node)
-    let parent = a:node.parent
-    while g:NERDTreeCascadeOpenSingleChildDir && !parent.isRoot()
-        let childNodes = parent.getVisibleChildren()
-        if len(childNodes) == 1 && childNodes[0].path.isDirectory
-            let parent = parent.parent
-        else
-            break
-        endif
-    endwhile
-    if parent ==# {} || parent.isRoot()
-        call nerdtree#echo("cannot close tree root")
-    else
-        call parent.close()
-        call b:NERDTree.render()
-        call parent.putCursorHere(0, 0)
+
+    if a:node.isRoot()
+        call nerdtree#echo('cannot close parent of tree root')
+        return
+    endif
+
+    let l:parent = a:node.parent
+
+    if empty(l:parent) || l:parent.isRoot()
+        call nerdtree#echo('cannot close tree root')
+        return
     endif
+
+    while l:parent.isCascadable()
+        let l:parent = l:parent.parent
+    endwhile
+
+    call l:parent.close()
+    call b:NERDTree.render()
+    call l:parent.putCursorHere(0, 0)
 endfunction
 
 " FUNCTION: s:closeTreeWindow() {{{1