bug#30280: async-shell-command-display-buffer doesn't work anymore

[Juri Linkov]

0. emacs -Q
1. Eval: (setq async-shell-command-buffer 'new-buffer
               async-shell-command-display-buffer nil)
2. M-& sleep 3600
3. M-& sleep 3; echo "xyzzy"

At the step 2 “*Async Shell Command*” is not shown because of no output,
this is ok.

But at the step 3 “*Async Shell Command*<2>” is not shown after 3 seconds
when the output arrives, this is a regression.

It seems it is caused by commit#9f4f130 from bug#28997

PS: Also it doesn't work with less official configuration:
0. emacs -Q
1. Eval: (setq async-shell-command-display-buffer nil)
         (add-hook 'shell-mode-hook 'rename-uniquely)
2. M-& sleep 3600
3. M-& sleep 3; echo "xyzzy"