This logic is now only used by cmd_init_db(). setup_* functions do not
rely on it any more. Move all the logic to cmd_init_db() and turn
get_git_work_tree() into a simple function.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>