repo: add the flag -z as an alias for --format=nul

repo: add the flag -z as an alias for --format=nul

Other Git commands that have nul-terminated output (e.g. git-config,
git-status, git-ls-files) have a flag `-z` for using the null character
as the record separator.

Add the `-z` flag to git-repo-info as an alias for `--format=nul`,
making it consistent with the behavior of the other commands.

Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Mentored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>