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

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

The config value `brach.autoSetupMerge` is parsed in
`git_default_branch_config()` and stored in the global variable
`git_branch_track`. This global variable can cause unexpected behaviours
when multiple Git repos run in the the same process.

Move this value into `struct repo_config_values` which holds all values
parsed by `git_default_config()` and can be accessed per
repo via `git_default_config()`. This would retain the same
behaviours while achieving repository scoped access.

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>