Fix focussing previous buffer when closing NERDTree

Previously closing NERDTree while two windows were showing the same buffer would
focus the first window, which was not necessarily the previously active one.

Instead of obtaining the buffer ID of the previous buffer and mapping that to
the window ID (which is a 1:n mapping) we obtain the unique window ID and focus
the right window after closing NERDTree.

win_getid() and win_gotoid() are available from VIM 7.4.1557, so this patch is
conditional on that version.

Author: Martin Rubli

lib/nerdtree/nerdtree.vim

@@ -40,15 +40,23 @@ function! s:NERDTree.Close()
     if winnr("$") != 1
         if winnr() == s:NERDTree.GetWinNum()
             call nerdtree#exec("wincmd p")
-            let bufnr = bufnr("")
+            if v:version > 704 || (v:version == 704 && has('patch1557'))
+                let l:winid = win_getid()
+            else
+                let bufnr = bufnr("")
+            endif
             call nerdtree#exec("wincmd p")
         else
-            let bufnr = bufnr("")
+            let l:winid = win_getid()
         endif
 
         call nerdtree#exec(s:NERDTree.GetWinNum() . " wincmd w")
         close
-        call nerdtree#exec(bufwinnr(bufnr) . " wincmd w")
+        if v:version > 704 || (v:version == 704 && has('patch1557'))
+            call nerdtree#exec("call win_gotoid(" . l:winid . ")")
+        else
+            call nerdtree#exec(bufwinnr(bufnr) . " wincmd w")
+        endif
     else
         close
     endif