bug#44983: Truncate long lines of grep output

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Dmitry Gutov <dgutov@yandex.ru>,  44983@debbugs.gnu.org,  juri@linkov.net
> Date: Tue, 01 Dec 2020 17:46:33 +0100
> 
> > Not necessarily, because the truncated parts are still in the buffer,
> > and the display code which is slow in that case basically moves
> > through the buffer one character at a time in many cases.  Only some
> > specific scenarios (read: a small number of commands) can jump to the
> > next physical line disregarding the truncated parts.
> 
> But moving though the buffer is much faster than rendering it.

I meant moving in the likes of move_it_to.  These simulate display, so
they are almost as slow as rendering itself.