repo: add -z as an alias for --format=nul to git-repo-structure

repo: add -z as an alias for --format=nul to git-repo-structure

Other Git commands that have nul-terminated output, such as git-config,
git-status, git-ls-files, and git-repo-info have a flag `-z` for using
the null character as the record separator.

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

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>