store b:changedtick after setting &filetype

FileType autocmd handlers may modify the buffer, that is considered an
"internal change", not a "user change".

Don't set &filetype if we are not reloading the buffer.  It doesn't make
sense to trigger FileType autocmd handlers if we merely navigated to an
existing, loaded buffer.

ref #102

autoload/dirvish.vim

@@ -410,9 +410,9 @@ function! s:do_open(d, reload) abort
   call s:win_init()
   if a:reload || s:should_reload()
     call s:buf_render(b:dirvish._dir, get(b:dirvish, 'lastpath', ''))
+    setlocal filetype=dirvish
     let b:dirvish._c = b:changedtick
   endif
-  setlocal filetype=dirvish
 endfunction
 
 function! s:should_reload() abort