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

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@gmail.com>
> Cc: bzg@gnu.org,  16493@debbugs.gnu.org,  monnier@iro.umontreal.ca
> Date: Sat, 11 May 2019 11:27:55 -0400
> 
> > I'd rather we fixed the remove-from-invisibility-spec result than
> > start promoting (t).
> 
> Starting from t, (add-to-invisibility-spec 'foo) also has the weird
> result that it *reduces* the number of properties which cause
> invisibility (from infinity to 2).

Yes, but I don't see that as a problem.  buffer-invisibility-spec is a
filter, so adding to it could have that effect.