submodule--helper: remove repeated code in sync_submodule()

submodule--helper: remove repeated code in sync_submodule()

This part of `sync_submodule()` is doing the same thing that
`compute_submodule_clone_url()` is doing. Let's reuse that helper here.

Note that this change adds a small overhead where we allocate and free
the 'remote' twice, but that is a small price to pay for the higher
level of abstraction we get.

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