repository: introduce repo_config_values_clear()

repository: introduce repo_config_values_clear()

As part of the ongoing libification effort, dynamically allocated
global configuration variables are being moved into
'struct repo_config_values'. To prevent memory leaks, we need a
destructor to free these heap-allocated variables when a repository
instance is torn down.

Introduce 'repo_config_values_clear()' in environment.c and invoke it
from 'repo_clear()' in repository.c. As a starting point, update this
new function to handle the cleanup of 'attributes_file'.

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>