Fix a menu command that breaks under 'shellslash'

The (l) menu command breaks on Windows systems when 'shellslash' is
set.  This is due to the fact that the menu item uses a hard coded
shell command, thus relying on the use of the default Windows shell
without the 'shellslash' setting.

The pattern used for the fix is localized to the problem function.
However, this technique could easily be abstracted into its own
function to execute Windows shell commands with the default shell
throughout the NERDTree codebase.

nerdtree_plugin/fs_menu.vim

@@ -212,14 +212,40 @@ endfunction
 
 " FUNCTION: NERDTreeListNodeWin32() {{{1
 function! NERDTreeListNodeWin32()
-    let treenode = g:NERDTreeFileNode.GetSelected()
-    if treenode != {}
-        let metadata = split(system('DIR /Q ' . shellescape(treenode.path.str()) . ' | FINDSTR "^[012][0-9]/[0-3][0-9]/[12][0-9][0-9][0-9]"'), '\n')
-        call nerdtree#echo(metadata[0])
-    else
-        call nerdtree#echo("No information avaialable")
+    let l:node = g:NERDTreeFileNode.GetSelected()
+
+    if !empty(l:node)
+
+        let l:save_shell = &shell
+        set shell&
+
+        if exists('+shellslash')
+            let l:save_shellslash = &shellslash
+            set noshellslash
         endif
 
+        let l:command = 'DIR /Q '
+                    \ . shellescape(l:node.path.str())
+                    \ . ' | FINDSTR "^[012][0-9]/[0-3][0-9]/[12][0-9][0-9][0-9]"'
+
+        let l:metadata = systemlist(l:command)
+
+        if v:shell_error == 0
+            call nerdtree#echo(l:metadata[0])
+        else
+            call nerdtree#echoError('shell command failed')
+        endif
+
+        let &shell = l:save_shell
+
+        if exists('+shellslash')
+            let &shellslash = l:save_shellslash
+        endif
+
+        return
+    endif
+
+    call nerdtree#echo('node not recognized')
 endfunction
 
 " FUNCTION: NERDTreeCopyNode() {{{1