Revert "remove NERDTree.previousBuf() and related code"

This reverts commit 2027ae03db.

lib/nerdtree/creator.vim

@@ -92,11 +92,14 @@ function! s:Creator.createWindowTree(dir)
     "we want the directory buffer to disappear when we do the :edit below
     setlocal bufhidden=wipe
 
+    let previousBuf = expand("#")
+
     "we need a unique name for each window tree buffer to ensure they are
     "all independent
     exec g:NERDTreeCreatePrefix . " edit " . self._nextBufferName()
 
     call self._createNERDTree(path, "window")
+    let b:NERDTree._previousBuf = bufnr(previousBuf)
     call self._setCommonBufOptions()
 
     call b:NERDTree.render()