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

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Tue, 5 Apr 2016 15:54:35 +0300
> Cc: 23179@debbugs.gnu.org
> 
> On 04/05/2016 08:43 AM, Anders Lindgren wrote:
> 
> > I gave it a test. The result looks good, and all matches are highlighted
> > (regardless if the files are loaded or not). Unfortunately, it comes
> > with a hefty overhead. The time increased from 7.5 s to 10.4 s.
> 
> That's too bad. Although the difference might be smaller with other 
> major modes than CC Mode, where syntax-propertize, currently called 
> anyway, is not a no-op.

I personally don't consider a 30% slow-down as significant, but if we
think others might disagree, perhaps we should have this as an
optional behavior.