environment: move "pack_compression_level" into `struct repo_config_values`

environment: move "pack_compression_level" into `struct repo_config_values`

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

Store it instead in `repo_config_values`, where eagerly‑parsed
repository configuration lives. `pack_compression_level` is parsed
eagerly because it influences packfile compression, a core operation
where a lazy parse could cause inconsistent behavior and hamper
libification. This preserves the existing eager‑parsing 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.

The type remains `int` as it represents a numeric compression level,
not a boolean toggle.

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>