environment: move autorebase into repo_config_values

environment: move autorebase into repo_config_values

The global variable 'autorebase' dictates whether a newly created
branch should be configured to automatically rebase by default.
Move it into 'struct repo_config_values' to continue the
libification effort.

The 'enum rebase_setup_type' definition is moved higher up in
'environment.h' so that it is visible to the repository-specific
structure. The default state AUTOREBASE_NEVER is now correctly
initialized in 'repo_config_values_init()'.

Configuration parsing in 'git_default_branch_config()' is updated to
write directly to the repository's configuration instance.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ayush Chandekar <redacted>
Mentored-by: Olamide Caleb Bello <redacted>
Signed-off-by: Tian Yuchen <redacted>
Signed-off-by: Junio C Hamano <redacted>