environment: environment: stop using core.sparseCheckout globally

environment: environment: stop using core.sparseCheckout globally

The config value `core.sparseCheckout` is parsed in
`git_default_core_config()` and stored globally in
`core_appy_sparse_checkout`. This could cause unintended behaviours
when different Git repositories running in the same process access this
variable.

Move the parsed value into `struct repo_config_values` which holds all the
values parsed by `git_default_config()` and can be accessed
per repo via `git_default_config()`. This retains current
behaviours while achieving the repository scoped access.

Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
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>