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

[Juri Linkov]

>>    M-x project-find-regexp
>>
>>      4. shouldn't it support an optional more compact grep-like output
>>         layout as well (where file names are on the same line with matches)?
>
> You can implement that via a custom xref-show-xrefs-function. Not trivial,
> but not a huge amount of work either.

A new problem:

6. M-x project-find-regexp RET -- RET

   finds nothing whereas I can see there are many places with
   such string “--” in project files.