bug#48925: [PATCH] Set `minibuffer-completion-*` variables buffer-locally in a few more places

[Stefan Monnier]

> Moving read-string to subr.el means the function will be unavailable
> during loadup until subr.elc is loaded.

I believe this should not be a problem: `read-string` is only used for
interaction with the user so it's never used until much later than the
load of `subr.el` (it's not used during bootstrap).

> What is awkward to do in C?

We don't have anby facility to create closures from C, so we'd basically
have to call an ELisp function to create the closure.


        Stefan