bug#45474: Icomplete exhibiting in recursive minibuffer when it shouldn’t

[Gregory Heytings]

> I'm more and more thinking that we should really bite the bullet and go 
> for a "really buffer-local" setup, such as in the patch below.
>
> I'd be surprised if it doesn't introduce backward incompatibilities, but 
> at least it's "The Right Thing" to do.

Well, that's what my patch does, too, except that it delays the backward 
incompatilibites during one Emacs major release...  (I did not include 
minibuffer-completion-predicate and minibuffer-completion-confirm because 
they haven't been discussed yet, but they are easy to add.)