bug#65928: [PATCH] Introduce 'project-save-buffers'

[Juri Linkov]

>> It could be bound to 'C-x p C-x s'.

After more thoughts, it's not the best keybinding:
the global 'C-x s' could be used to save all buffers
in another project with 'C-x p p ... C-x s'
implemented in bug#63648.

> What's the binding in projectile?  All else being equal, I'd suggest we
> use the same one.

Maybe Projectile's binding 'C-x p S' is not too bad.

> Would something like
>     (defalias 'project-save-buffers #'save-some-buffers-root)
> do the job?

Rather something more like this will do the job:

```
(keymap-set project-prefix-map "S"
            (lambda (&optional arg)
              (interactive "P")
              (let ((save-some-buffers-default-predicate 
'save-some-buffers-root))
                (save-some-buffers arg))))
```