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

[Troy Brown <brownts@troybrown.dev>]

On Sun, Sep 8, 2024 at 2:28 PM Daniel Mendler <mail@daniel-mendler.de> wrote:
>
> Juri Linkov <juri@linkov.net> writes:
>
> > Maybe Daniel (Cc:ed) could help us understand
> > why 'completion--insert-horizontal' doesn't display groups.
>
> If I remember correctly, I didn't add group titles to the horizontal
> formatting in order to keep the display more dense, with the goal of
> fitting as many candidates on the screen as possible. In my Vertico
> package, I am using a similar strategy, where group titles are not shown
> for the flat and grid display modes. In any case, there shouldn't be a
> technical reason which prevents us from showing the titles also for the
> horizontal display.

It appears to be this line in `completion--insert-horizontal` which is
causing all the trouble.  When I comment that out, I get the output
I'm expecting.  Looking at the difference between
`completion--insert-horizontal` and `completion--insert-vertical`,
that check happens after the check for `group-fun`.  Maybe
`completion--insert-horizontal` should be restructured similarly?

```elisp
      (unless (equal last-string str) ; Remove (consecutive) duplicates.
```