submodule--helper: refactor resolve_relative_url() helper

submodule--helper: refactor resolve_relative_url() helper

Refactor the helper function to resolve a relative url, by reusing the
existing `compute_submodule_clone_url()` function.

`compute_submodule_clone_url()` performs the same work that
`resolve_relative_url()` is doing, so we eliminate this code repetition
by moving the former function's definition up, and calling it inside
`resolve_relative_url()`.

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>