[Buffers] Increase tabstop according to the string length of bufno

e.g. vim {1..1000} +'silent 1bd'

Fix #1237

autoload/fzf/vim.vim

@@ -730,7 +730,7 @@ function! fzf#vim#buffers(...)
         \ [a:1, a:000[1:]] : ['', a:000]
   let sorted = fzf#vim#_buflisted_sorted()
   let header_lines = '--header-lines=' . (bufnr('') == get(sorted, 0, 0) ? 1 : 0)
-  let tabstop = len(sorted) >= 1000 ? 9 : 8
+  let tabstop = len(max(sorted)) >= 4 ? 9 : 8
   return s:fzf('buffers', {
   \ 'source':  map(sorted, 'fzf#vim#_format_buffer(v:val)'),
   \ 'sink*':   s:function('s:bufopen'),