t/t1517: automate `git subcmd -h` tests outside a repository

t/t1517: automate `git subcmd -h` tests outside a repository

Replace manual `-h` tests with a loop over all subcommands using
`git --list-cmds=main`. This ensures consistent coverage of `-h`
behavior outside a repo and future-proofs the test by covering
new commands automatically.

Known exceptions are skipped or marked as expected failures.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>