Use git_config_early() instead of git_config() during repo setup

Use git_config_early() instead of git_config() during repo setup

When git_config() is called, either git_dir has already been set (by
$GIT_DIR env or set_git_dir()), or it will default git_dir to ".git".

git_config_early() gives setup functions more freedom because it does
not require git_dir. Give it a config path, it will happily examine
it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>