environment: move "precomposed_unicode" into `struct repo_config_values`

environment: move "precomposed_unicode" into `struct repo_config_values`

The `core.precomposeunicode` configuration is currently stored in the
global variable `precomposed_unicode`, 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.precomposeunicode` is parsed
eagerly because it controls Unicode path normalization on macOS,
a fundamental filesystem‑level behavior that many operations depend
on; a lazy parse could lead to inconsistent results and hamper
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>