Handle buffer numbers correctly in autocmd

As far as I know this has never caused a problem but it is good to get
it right.

plugin/rooter.vim

@@ -64,18 +64,20 @@ command! -bar RooterToggle call <SID>toggle()
 
 augroup rooter
   autocmd!
-  autocmd VimEnter,BufReadPost,BufEnter * nested if !g:rooter_manual_only | Rooter | endif
-  autocmd BufWritePost * nested if !g:rooter_manual_only | call setbufvar('%', 'rootDir', '') | Rooter | endif
+  autocmd VimEnter,BufReadPost,BufEnter * nested if !g:rooter_manual_only | call <SID>rooter(+expand('<abuf>')) | endif
+  autocmd BufWritePost * nested if !g:rooter_manual_only | call setbufvar(+expand('<abuf>'), 'rootDir', '') | call <SID>rooter(+expand('<abuf>')) | endif
 augroup END
 
 
-function! s:rooter()
+function! s:rooter(...)
   if !s:activate() | return | endif
 
-  let root = getbufvar('%', 'rootDir')
+  let bufnr = a:0 ? a:1 : '%'
+
+  let root = getbufvar(bufnr, 'rootDir')
   if empty(root)
     let root = s:root()
-    call setbufvar('%', 'rootDir', root)
+    call setbufvar(bufnr, 'rootDir', root)
   endif
 
   if empty(root)