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

[Eli Zaretskii]

> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 4 Apr 2016 20:25:51 +0300
> Cc: 23179@debbugs.gnu.org
> 
> >     You've never used e.g. xref-find-references?
> >
> > No. I went into this with the eyes of an existing tags user, and
> > reported the problems I saw.
> 
> etags has no counterpart for this command. Note, though, that the 
> default implementation relies on the Project package.

It does?  But it works for me (searching C symbols in Emacs sources)
with no extra preparations.  What am I missing?