environment: move "branch.autoSetupMerge" into `struct repo_config_values`

environment: move "branch.autoSetupMerge" into `struct repo_config_values`

The config value `branch.autoSetupMerge` is parsed in
`git_default_branch_config()` and stored in the global variable
`git_branch_track`. This global variable can be overwritten
by another repository when multiple Git repos run in the the same process.

Move this value into `struct repo_config_values` in the_repository to
retain current behaviours and move towards libifying Git.
Since the variable is no longer a global variable, it has been renamed to
`branch_track` in the struct `repo_config_values`.

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