environment: move push_default into repo_config_values
The global variable 'push_default' specifies the default behavior of 'git push' when no explicit refspec is provided. Move 'push_default' into 'struct repo_config_values' to continue the libification effort. While 'enum push_default_type' ideally belongs in 'remote.h', moving it there introduces a circular dependency chain: remote.h -> hash.h -> repository.h -> environment.h. Therefore, the enum definition is kept in 'environment.h' just above 'struct repo_config_values' with a NEEDSWORK comment for future cleanup. Modify the configuration parsing in environment.c to update the per-repository structure directly, and update caller across the codebase to access the value via 'repo_config_values()'. 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>