It is unnecessary to call `transient--show' here because it will
later happen anyway when `transient--post-command' calls either
`transient--refresh-transient' or `transient--redisplay'.

Calling `transient--show' here also meant that it could not take any
state changes into account that the invoked command made, which is
relevant if `refresh-suffixes' is non-nil and those changes effect
the result of, e.g., suffix predicates.

Closes #397.

This could happen, for example, if a menu only has a single suffix and
invoking that suffix causes it to be removed from the menu.  That is a
rather contrived use-case, but see #397.

Emacs commit 60c10b40d5b addresses this issued.  That hasn't landed
on master yet (and so the hash is likely to change), so let's copy
the message here:

> Make C-g in command loops consistently invoke quit keybinding
>
> C-g in command loops could unpredictably either signal an error or
> invoke the quit_char keybinding, depending on the relative timing
> of input and redisplay.  This change ensures C-g always runs the
> quit_char keybinding by wrapping read_char to check for quit_flag
> after reading input.

For older Emacs versions we have to work around the issue using this
new advice, approximately as suggested by Daniel Colascione at
https://github.com/magit/transient/commit/45fbefdc5b112f0a15cd93657.

The `unreadp' variable is an additional safety net, which should not
be necessary.  It would kick in if the unread C-g again causesd quit
to be signaled, or if somehow our transient keymap got disabled,
without this advice also getting disabled.

Also see #388.

For class constructors and `clone', if a keyword argument is specified
multiple times, the latter instance wins, which in our case was the
outdated one.

As a result, it was impossible to change the level of a suffix if that
was set inline, in the prefix definition.

Fixes [1: bcc0bf83].

1: 2025-05-16 bcc0bf836808e156262d9ff631be7d91a0ec201d
   Improve format of layout and handling of included groups