submodule--helper: allow setting superprefix for init_submodule()

submodule--helper: allow setting superprefix for init_submodule()

We allow callers of the `init_submodule()` function to optionally
override the superprefix from the environment.

We need to enable this option because in our conversion of the update
command that will follow, the '--init' option will be handled through
this API. We will need to change the superprefix at that time to ensure
the display paths show correctly in the output messages.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>