bug#70511: Option to grep into compressed files

[Mary]

> For a while, new options were getting added to GNU tar frequently in order
> to allow you to do things like
> 
> compress -dc | tar xf -
> zcat | tar xf -
> bzcat | tar xf -
> lzcat | tar xf -
> 
> etc., but just using the single tar invocation without (explicitly
> running) an external compression program. The current ones are (in
> alphabetical order in the man page, not historical order of when they
> were added)
> 
> -j, --bzip2
> Filter the archive through bzip2(1).
> 
> -J, --xz
> Filter the archive through xz(1).
> 
> --lzip Filter the archive through lzip(1).
> 
> --lzma Filter the archive through lzma(1).
> 
> --lzop Filter the archive through lzop(1).
> 
> -z, --gzip, --gunzip, --ungzip
> Filter the archive through gzip(1).
> 
> -Z, --compress, --uncompress
> Filter the archive through compress(1).
> 
> --zstd Filter the archive through zstd(1).
> 
> Wow, eight specific forms of compression! But a newer functionality
> in GNU tar is
> 
> -a, --auto-compress
> Use archive suffix to determine the compression program.
> 
> and something like that (apparently also looking at the file header)
> is now the default.
> 
> It's weird to me to imagine having all of that functionality in grep,
> but maybe all of the functionality that was put into tar for this could
> become a separate standalone program?

GNU tar also supports `-I, --use-compress-program=PROG   filter through PROG 
(must accept -d)`, which is one of the reasons I thought it would be relevant 
to add a similar option to grep.