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

[Juri Linkov]

> Being able to set `save-some-buffers-root' as the value of
> `save-some-buffers-default-predicate' is easy for users.
>
> The problem I see is that it hides the real nature of 
> `save-some-buffers-root':
> - it's not a predicate (as the docstring of 
> `save-some-buffers-default-predicate' suggests).
> - it's a function generating the default predicate.
>
> We can make the distinction (default predicate <-> func generating a default 
> predicate)
> more clear if we put the generating functions in a list.
>
> Then, we can restrict the allowed pred-fun to the elements inside such a list.
>
> I have played today with this quick-and-dirty patch:
> How do you think?

Nice idea, and it's better than what I proposed.
The only change that I suggest is instead of a list,
to put a special symbol on the symbol property,
for example:

  (put 'save-some-buffers-root 'higher-order-function t)
or simply
  (put 'save-some-buffers-root 'hof t)

A better name is appreciated.