Restore the cached _sortKey for faster processing.

Also add a new global variable to track when the g:NERDTreeSortOrder
changes. If it has been changed, or when the cached _sortKey value is
uninitialized, then calculate the sort key. This improves processing
speed over the previous commit, and allows on-the-fly changes to the
sort order, (without required vim to be restarted.)

lib/nerdtree/path.vim

@@ -392,6 +392,7 @@ endfunction
 " FUNCTION: Path.getSortKey() {{{1
 " returns a key used in compare function for sorting
 function! s:Path.getSortKey()
+    if !exists("self._sortKey") || g:NERDTreeSortOrder !=# g:NERDTreeOldSortOrder
         let path = self.getLastPathComponent(1)
         if !g:NERDTreeSortHiddenFirst
             let path = substitute(path, '^[._]', '', '')
@@ -404,6 +405,7 @@ function! s:Path.getSortKey()
         else
             let self._sortKey = [self.getSortOrderIndex()] + self._splitChunks(path)
         endif
+    endif
 
     return self._sortKey
 endfunction

lib/nerdtree/tree_dir_node.vim

@@ -612,6 +612,7 @@ function! s:TreeDirNode.sortChildren()
     endif
     let CompareFunc = function("nerdtree#compareNodesBySortKey")
     call sort(self.children, CompareFunc)
+    let g:NERDTreeOldSortOrder = g:NERDTreeSortOrder
 endfunction
 
 " FUNCTION: TreeDirNode.toggleOpen([options]) {{{1

plugin/NERD_tree.vim

@@ -82,6 +82,7 @@ call s:initVariable("g:NERDTreeCascadeSingleChildDir", 1)
 if !exists("g:NERDTreeSortOrder")
     let g:NERDTreeSortOrder = ['\/$', '*', '\.swp$',  '\.bak$', '\~$']
 endif
+let g:NERDTreeOldSortOrder = []
 
 call s:initVariable("g:NERDTreeGlyphReadOnly", "RO")