bug#48572: 28.0.50; Add `passthrough` completion style to minibuffer.el

[Stefan Monnier]

> Sorry, I don't get it. What is the difference in calling the backend
> between the two cases:
>
> (1) passthrough-all-completions -> all-completions -> backend
>
> versus
>
> (2) passthrough-all-completions -> backend?

In case 2 you get to choose the arguments passed to the backend function.

> In case (2) you call the backend directly but then you also have to
> handle completion tables which are not functions and you end up

`passthrough` doesn't make any sense for non-function backends.

> Yes, I am aware that `all-completions` is the backend API. Is anything
> lost when going through `all-completions`?

The possibility to return completion candidates that aren't "prefix
completions" (which is the whole purpose of `passthrough`)?


        Stefan