Recenter automatically the window showing output buffer

[Tino Calancha]

Dear Emacs,

I am trying to do something apparently simple, but I cannot make it
work consistenly.

* Start a shell process which print out the string 'Hello!\n'.
* Set a simple sentinel to prevent the buffer from showing
  'Process *shell* finished'.

The output buffer consists of just 2 lines (second is empty).
The window doesn't display 'Hello!': it shows the output buffer 
from line 2.

--8<-----------------------------cut here---------------start------------->8---
(let* ((buf (generate-new-buffer "*foo*"))
      (proc (start-process-shell-command
             "*shell*" buf "printf 'Hello!\n'"))
      (sentinel (lambda (process status)
                  (message "process '%s' %s" (process-name process) status))))
  (set-process-sentinel proc sentinel)
  (display-buffer buf))
--8<-----------------------------cut here---------------start------------->8---

I've tried adding after `display-buffer' the following:
  (let ((recenter-redisplay t)) ; Force a redisplay
    (with-selected-window (get-buffer-window buf) (recenter))))

But it doesn't really make a difference.

How can I achieve the recenter in buf?

Thank you,
Tino