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 <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>