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

[Eli Zaretskii]

> From: John Wiegley <jwiegley@gmail.com>
> Date: Sun, 03 Apr 2016 10:31:54 -0700
> Cc: 23179@debbugs.gnu.org, Anders Lindgren <andlind@gmail.com>
> 
> The subject of the bug, and M-, having a potentially surprising new
> non-behavior. If it's just a matter of right configuration, then we should
> have that configuration be the default.
> 
> Tags is a really old service that many people have grown to depend on, so we
> shouldn't be taking anything away from people by default who switch to
> Emacs 25. Otherwise, there may be many irate bugs coming.

We've made a consistent and conscious effort to switch from the
tags-loop-continue UI to the xref UI.  The only command that still
needs that and doesn't have an xref-based equivalent is tags-search.
If we add such an equivalent command, the issue with
tags-loop-continue will only arise if the user uses the obsolete
commands (in which case they will probably rebind the keys anyway).

When all of these commands have xref UI, the M-, binding to
tags-loop-continue is no longer necessary because the command is
unnecessary.