bug#25482: 26.0.50; Allow setting `query-replace-from-to-separator` to nil

[Eli Zaretskii]

> From: Thierry Volpiatto <thierry.volpiatto@gmail.com>
> Cc: 25482@debbugs.gnu.org
> Date: Sat, 28 Jan 2017 12:23:54 +0100
> 
> > Yes, the external packages need to adapt to this, if they want to use
> > the internal features.  I don't think this can be avoided in general.
> 
> What can be avoided is writing complex code when simple code can be used
> for the same result.

Maybe so, but that ship has sailed more than 2 years ago, and was
preceded by a long discussion.  I didn't read all of it now, but
AFAIU, the reason for using a character with a display property was to
make the separator not editable when the user edits the history.  So
that design decision and the resulting code we have now had some
justification, and any replacement will have to support the same
features, which I think a simple string will not do.