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

[Juri Linkov]

>> I tried to run rgrep in `emacs -Q' and see that its output is unsorted
>> indeed.  I forgot that I customized the sorting of the output long ago:
>>
>> (setq grep-find-template
>> "find <D> <X> -type f <F> -print0 | sort -z | xargs -0 -e grep <C> --color 
>> -inH -e <R>")
>>                                      =======
>
> I see.
>
> But actually, I looked at the code again, and there's little reason not to
> sort in the current implementation. Please try and see if doing that in
> project--files-in-directory gives any kind of perceptible slowdown.

No perceptible slowdown, thanks.

> Doing it in Lisp seems to be the fastest choice (benchmark still shows
> a bit of a difference if I do that via "| sort -z"):

And also avoids any possible incompatibilities in options of the external 
command.