bug#73117: 30.0.90; Imenu missing entries when flattening by group

[Juri Linkov <juri@linkov.net>]

> Maybe using `(setopt completion-auto-help t)`
> and hitting TAB is a better method.

Or just using `(setopt imenu-eager-completion-buffer nil)`
because otherwise with its default value you need to hit
'?' (minibuffer-completion-help) instead of TAB
to show the completions buffer with two identical
completions in different groups.

> I did try adding your suggested settings to the above code snippet,
> but they did not seem to improve anything with regards to what shows
> up in the completions buffer:
>
> ```
> (setopt completions-group t)
> (setopt minibuffer-visible-completions t)
> ```

I'm so sorry, I forgot to check with `emacs -Q`, and there is
an additional important setting that affects the groups:

```
(setopt completions-group t)
(setopt completions-format 'vertical)
```

The default value of 'completions-format' is 'horizontal'
that doesn't support groups, and I don't know why.

Maybe Daniel (Cc:ed) could help us understand
why 'completion--insert-horizontal' doesn't display groups.