submodule--helper: introduce get_submodule_displaypath()

submodule--helper: introduce get_submodule_displaypath()

Introduce function get_submodule_displaypath() to replace the code
occurring in submodule_init() for generating displaypath of the
submodule with a call to it.

This new function will also be used in other parts of the system
in later patches.

Mentored-by: Christian Couder <redacted>
Mentored-by: Stefan Beller <redacted>
Signed-off-by: Prathamesh Chavan <redacted>
Signed-off-by: Junio C Hamano <redacted>