environment: move "check_stat" into `struct repo_config_values`

environment: move "check_stat" into `struct repo_config_values`

The `core.checkstat` configuration is currently stored in the global
variable `check_stat`, which makes it shared across repository
instances within a single process.

Store it instead in `repo_config_values`, where eagerly‑parsed
repository configuration lives. `core.checkstat` is parsed eagerly
because it controls how `match_stat_data()` and related functions
decide file freshness; a lazy parse could lead to unexpected
behavior or complicate libification. This preserves the existing
eager‑parsing behavior while tying the value to the repository it
was read from, 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>