bug#51029: 28.0.60; Follow-up to Bug#51029: ibuffer-shrink-to-fit does not call fit-window-to-buffer

[Y . E .]

This is a follow-up to Bug#51029.
It seems there's more to fix.

Steps to reproduce:

1. emacs -Q
2. (setq ibuffer-default-shrink-to-minimum-size t)
3. M-x ibuffer-other-window
4. *Ibuffer* isn't shrinked

[Because ibuffer-auto-mode is not bound and hence fit-window-to-buffer
is not called.]

Thank you.