bug#30503: 27.0.50; allow hiding M-x grep command line

[Eli Zaretskii <eliz@gnu.org>]

> From: Juri Linkov <juri@linkov.net>
> Date: Tue, 20 Feb 2018 22:54:34 +0200
> Cc: 30503@debbugs.gnu.org
> 
> > Could we provide an option to hide the barrage of ignored directories
> > and files?  It might also be worth adding a text button or keybinding
> > to toggle their visibility interactively.
> 
> Yes, this is a real problem.  Even though I set truncate-lines to t,
> often there is a need to see grep switches at the end of the
> command line.  So there is a patch to hide uninteresting parts
> under a button like is used to hide part of output by
> elisp-last-sexp-toggle-display.  It supports rgrep, lgrep and zrgrep.

Thanks, but where's the option to hide the long list?  It looks like
this patch changes the behavior unconditionally, and makes it
strikingly different from the previous behavior.

In any case, this needs to be documented in NEWS and the manual, I
think.