repo: use [--format=... | -z] instead of [-z] in git-repo-info synopsis

repo: use [--format=... | -z] instead of [-z] in git-repo-info synopsis

The flag -z is only an alias for --format=null and even though --format
and -z can be used together and repeated, only the last one is
considered.

Replace `[-z]` in the synopsis of git-repo-info by
`[--format=... | -z]`, expliciting that the use of one of those flags
replace the other.

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