bug#16493: 24.3.50; (setq search-invisible t) is useless, let's allow to turn visible-mode temporarily on

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Tue, 30 Apr 2019 08:45:54 -0400
> Cc: Bastien <bzg@gnu.org>, 16493@debbugs.gnu.org
> 
> You need to use `add-to-invisibility-spec` before that works.  E.g.: 
> 
>     (add-to-invisibility-spec 'dummy)
>     
> that's because of a quirk of the way we treat `invisible` and the
> `buffer-invisibility-spec`.
> 
> 
>         Stefan
>         
> 
> PS: To fix this quirk, we should change buffer-invisibility-spec's
> default value to nil (or `(t)`, maybe) instead of t.

That'd be confusing, IMO.  If we need to change some of the semantics
of buffer-invisibility-spec, I'd prefer to introduce some new form of
value which would mean "override any other invisibility properties".