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.)

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")