bug#72156: 31.0.50; defcustom set issue

[Eli Zaretskii <eliz@gnu.org>]

merge 72156 72404
thanks

> Cc: 72156@debbugs.gnu.org
> Date: Sat, 20 Jul 2024 11:50:47 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > Date: Wed, 17 Jul 2024 14:08:59 +0200
> > From:  Ergus via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> > 
> > I have this defcustom:
> > 
> > ```
> > (defcustom gtags-mode-features '(control meta)
> >   "HELLO WORLD"
> >   :type '(set (const control)
> >               (const meta)
> >               (const shift)
> >               (const hyper)
> >               (const super)
> >               (const alt)))
> > ```
> > 
> > When I try the customize-variable I get this:
> > 
> > ```
> > Hide Gtags Mode Features:
> > Set:
> > [X] control
> > [X] meta
> > [ ] shift
> > [ ] hyper
> > [ ] super
> > [ ] alt
> >    [ State ]: EDITED, shown value does not take effect until you set or save it.
> >    HELLO WORLD
> > Groups: [Gtags Mode]
> > ```
> > 
> > But, if I do the next change and save
> > 
> > 
> > ```
> > Hide Gtags Mode Features:
> > Set:
> > [X] control
> > [X] meta
> > [X] shift
> > [ ] hyper
> > [ ] super
> > [ ] alt
> >    [ State ]: EDITED, shown value does not take effect until you set or save it.
> >    HELLO WORLD
> > Groups: [Gtags Mode]
> > ```
> > 
> > The customize variable value saved is:
> > 
> > '(gtags-mode-features3 '(shift))
> > 
> > it only saved the "new" value and ignores the previous
> > existing ones.
> > 
> > When I restart and check the variable's value I get as expected '(shift)
> > and lost the control and meta.
> > 
> > Going to customize-variable again I get:
> > 
> > ```
> > Hide Gtags Mode Features:
> > Set:
> > [ ] control
> > [ ] meta
> > [X] shift
> > [ ] hyper
> > [ ] super
> > [ ] alt
> >    [ State ]: EDITED, shown value does not take effect until you set or save it.
> >    HELLO WORLD
> > Groups: [Gtags Mode]
> > ```
> > 
> > Consistent with the variable value, but inconsistent with what was
> > actually saved before.
> > 
> > IF instead of selecting a new value I un-select some, then variable
> > saves nil and will be nil.
> 
> This seems to be some bug in handling the 'set' type of defcustoms.  I
> see the same problem when customizing completions-merging-modes from
> completion.el.  Perhaps Mauro (CC'ed) could chime in and suggest how
> to solve this.

This actually seems to be the same problem as bug#72404, since the fix
proposed by Stephen there fixes this one as well.  So I'm merging them.