environment: move excludes_file into repo_config_values

environment: move excludes_file into repo_config_values

The global variable 'excludes_file' is used to track the path to the
global ignore file. If this variable is NULL,
'setup_standard_excludes()' in 'dir.c' forcefully evaluates and assigns
the XDG default path to it.

Continue the libification effort by encapsulating this lazy-loading
fallback logic into a proper getter and moving the variable into
'struct repo_config_values'.

Since 'excludes_file' is a dynamically allocated string, it requires
proper heap memory management. It is safely freed using the newly
introduced 'repo_config_values_clear()' function when the repository
is torn down.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>