environment: move "core_sparse_checkout_cone" into `struct repo_config_values`

environment: move "core_sparse_checkout_cone" into `struct repo_config_values`

The `core.sparseCheckoutCone` configuration was previously stored in an
uninitialized global `int` variable, risking cross‑repository state
leakage.

Move it into `repo_config_values`, where eagerly‑parsed repository
configuration lives. `core.sparseCheckoutCone` is parsed eagerly
because it determines the fundamental sparse‑checkout mode and is
consulted very early during repository setup; a lazy parse could
leave the sparse‑checkout state undefined and complicate
libification. This preserves the existing behavior while tying the
value to the repository from which it was read, avoiding cross‑
repository state leakage and continuing the effort to reduce reliance
on global configuration state.

Update all references to use `repo_config_values()`.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Signed-off-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>