really fix the previous buffer issue when closing a tree win

So :bprev was failing to jump back to the right buffer. I dont fully
understand this yet, but I'm fairly sure this is because there is a
magic <directory> buffer that we delete when creating a wintree :-/

Anyway, we are explicitly storing the prev buffer again - and updating
it when reusing treewins. It's worky, but there may be a superior way...

lib/nerdtree/nerdtree.vim

@@ -186,6 +186,10 @@ function! s:NERDTree.previousBuf()
     return self._previousBuf
 endfunction
 
+function! s:NERDTree.setPreviousBuf(bnum)
+    let self._previousBuf = a:bnum
+endfunction
+
 "FUNCTION: s:NERDTree.render() {{{1
 "A convenience function - since this is called often
 function! s:NERDTree.render()