environment: stop storing `core.attributesFile` globally

environment: stop storing `core.attributesFile` globally

The `core.attributeFile` config value is parsed in
git_default_core_config(), loaded eagerly and stored in the global
variable `git_attributes_file`. Storing this value in a global
variable can lead to it being overwritten by another repository when
more than one Git repository run in the same Git process.

Create a new struct `repo_config_values` to hold this value and
other repository dependent values parsed by `git_default_config()`.
This will ensure the current behaviour remains the same while also
enabling the libification of Git.

An accessor function 'repo_config_values()' s created to ensure
that we do not access an uninitialized repository, or an instance
of a different repository than the current one.

Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Usman Akinyemi <usmanakinyemi202@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Olamide Caleb Bello <belkid98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>