bug#46374: 28.0.50; Ask me to save buffers only if they are under callers dir

[Eli Zaretskii]

> From: Tino Calancha <tino.calancha@gmail.com>
> Date: Wed, 28 Apr 2021 21:31:25 +0200
> Cc: 46374@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>,
>  Quách Mỹ Uyên Nhi
>  <uyennhi.qm@gmail.com>
> 
> Juri Linkov <juri@linkov.net> writes:
> 
> +(defcustom save-some-buffers-default-predicate save-some-buffers-default-fun
> +  "Generator function of the default predicate for `save-some-buffers'.
> +
> +It must be a function with no arguments that returns a predicate.
> +This predicate is called (with no parameters) from the buffer to be
> +saved.

Reading this, I'm confused: a function that returns a predicate?  If
this is literally so, why do we need this two-step approach? why not
have the value _be_ a predicate function?