bug#6991: Please keep bytecode out of *Backtrace* buffers

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Sat, 26 Nov 2016 19:33:30 -0500
> Cc: Eli Zaretskii <eliz@gnu.org>, Juanma Barranquero <lekktu@gmail.com>, John 
> Wiegley <johnw@gnu.org>, 
>       Stefan Monnier <monnier@iro.umontreal.ca>, 6991@debbugs.gnu.org, 
>       Lars Magne Ingebrigtsen <larsi@gnus.org>
> 
> I think it might make sense to hide the bytecode with display
> properties, but keep the text itself so that it's not lost when copied
> to an email.

Text properties won't prevent the hidden text from being copied.