bug#31558: 27.0; `custom-file' settings messed up by Emacs 27

[Drew Adams <drew.adams@oracle.com>]

Admittedly, this is probably not a problem that others will run into
often.  But it is quite annoying for me.

I think the problem was introduced in Emacs 27, but I'm not positive.

This is the problem:

If I update a user option and then save the new value, it is saved to my
`custom-file', as usual.  But thereafter my `custom-file' cannot be
loaded by Emacs 20, because of these two entries that have been updated:

 '(tramp-default-method "ftp" nil (tramp))
 '(tramp-verbose 9 nil (tramp))

I did not modify or ask to save those two options.  Their values remain
the same as they were.  What has happened is that Emacs has now added
"nil (tramp)", and those additional elements in the list make Emacs 20
choke, because library `tramp' does not exist in Emacs 20.

Signaling: (file-error "Cannot open load file" "tramp")
  require(tramp)
  mapcar(require (tramp))
  custom-set-variables(... (tramp-debug-buffer t)
                           (tramp-default-method "ftp" nil (tramp))
                           (tramp-verbose 9 nil (tramp)) ...)

But why must Emacs now add that "nil (tramp)" to my custom settings?
Must defining the option value require the library?

I have no other case of an option where such a require argument is
inserted.  And I need not use Tramp at all, for these options to get
updated this way.

Should Emacs (e.g. 27) be doing that systematically?  If it should, then
what's the best way for me to prevent it from doing that, for my use
case?

(In fact, in Emacs 20, the doc string for `custom-set-variables' does
not even mention the possibility of an entry having a 4th argument,
REQUEST, but the code for that function does handle it, as
`custom-requests'.)

In GNU Emacs 27.0.50 (build 3, x86_64-w64-mingw32)
 of 2018-03-21
Repository revision: e70d0c9e66d7a8609450b2889869d16aeb0363b5
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install -C 'CFLAGS=-O2 -static -g3''