bug#79170: Please make amhello-1.0.tar.gz reproducible

Hi Jelle - thanks for the report.

    The owner/guid could be set --owner=root:0 --group=root:0

OK.

      and to get rid of the timestamp --mtime='@0' (Or alternatively a fixed 
    timestamp which can be set via SOURCE_DATE_EPOCH).

Do you have a specific suggestion for how to use SOURCE_DATE_EPOCH here?
I don't want to fake the time when all the files were created to 1970 or
anything else. Having real mtimes seems useful.

    Alternatively the timestamp could be excluded by hardcoding the tar 
    format to ustar which omits timestamps.

1) What timestamp does ustar omit? AFAIK, every tar format stores mtimes
(and it's a good thing that they do).

2) In the latest automake release, I believe amhello already is ustar,
because we changed automake's default to be that in 1.18.
   
$ zcat /.../latest/.../gnu/share/doc/automake/amhello-1.0.tar.gz | file -
/dev/stdin: POSIX tar archive

Thanks,
Karl