bug#52815: Editable grep buffer

[ndame]

> The alternative is to rewrite wgrep.el.

Haven't checked the code in occur, but  if propagating the
changes back to the original location is implemented there,
then I'd think the same code would work for grep too by adding
switching to the relevant file before applying the changes.

Maybe that code could be extracted and emacs could have a generic
mechanism for showing lines from other files in a buffer and
propagating changes there back to the original location.