builtin/submodule--helper: add gitdir command

builtin/submodule--helper: add gitdir command

This exposes the gitdir name computed by submodule_name_to_gitdir()
internally, to make it easier for users and tests to interact with it.

Next commit will add a gitdir configuration, so this helper can also be
used to easily query that config or validate any gitdir path the user
sets (submodule_name_to_git_dir now runs the validation logic, since
our previous commit).

Based-on-patch-by: Brandon Williams <bwilliams.eng@gmail.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>