bug#23179: 25.0.92; Restore `M-,' to continue etags search

[Eli Zaretskii]

> Cc: 23179@debbugs.gnu.org, andlind@gmail.com
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 5 Apr 2016 19:00:53 +0300
> 
> On 04/05/2016 06:56 PM, Eli Zaretskii wrote:
> 
> >> The idea is that the other UI should really be a new
> >> xref-show-xrefs-function, that's what that variable is there for (you
> >> can make it a defcustom). It would be better to avoid coupling the two UIs.
> >
> > If the difference will prove to be more significant, I can see the
> > point.
> 
> The difference will hopefully increase in the future. And then we'll be 
> happy that the user doesn't need to change their customization.

The defcustom can control also which function(s) are called.  Asking
users to customize options whose values are functions is something I
think we should avoid as much as possible.

> > I thought that if one invokes next-error immediately after xref
> > collected the matches, this danger is avoided.  Isn't that true?
> 
> Not necessarily (e.g. if you have a Grep buffer visible). And a 
> next-error-function-capable buffer can come up after one of the 
> next-error invocations.

I guess those who want the UI be hidden will have to consider this
caveat and deal with it, if and when it happens.  TANSTAAFL.