submodule--helper: add options for compute_submodule_clone_url()

submodule--helper: add options for compute_submodule_clone_url()

Let's modify the interface to `compute_submodule_clone_url()` function
by adding two more arguments, so that we can reuse this in various parts
of `submodule--helper.c` that follow a common pattern, which is--read
the remote url configuration of the superproject and then call
`relative_url()`.

This function is nearly identical to `resolve_relative_url()`, the only
difference being the extra warning message. We can add a quiet flag to
it, to suppress that warning when not needed, and then refactor
`resolve_relative_url()` by using this function, something we will do in
the next patch.

We also rename the local variable 'relurl' to avoid potential confusion
with the 'rel_url' parameter while we are at it.

Having this functionality factored out will be useful for converting the
rest of `submodule add` in subsequent patches.

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>