submodule--helper: refactor get_submodule_displaypath()

submodule--helper: refactor get_submodule_displaypath()

We create a function called `do_get_submodule_displaypath()` that
generates the display path required by several submodule functions, and
takes a custom superprefix parameter, instead of reading it from the
environment.

We then redefine the existing `get_submodule_displaypath()` function
as a call to this new function, where the superprefix is obtained from
the environment.

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>