config: use repo_ignore_case() to access core.ignorecase

config: use repo_ignore_case() to access core.ignorecase

Replace the accesses to the global 'ignore_case' variable with
calls to 'repo_ignore_case(the_repository)'. This step eliminates
the 'ignore_case' global state.

Note on compat/win32/path-utils.c:
To eliminate the global state, several helper functions
(e.g. 'win32_fspathncmp()') now read from
'repo_ignore_case(the_repository)'. While this introduces
dependency on 'repository.h' into the 'compat/', it avoids massive
refactoring of the signatures across the codebase.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Mentored-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Tian Yuchen <cat@malon.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>