The argument name "optional" may mislead the reader to think this
option could be NULL. But it can't be. While at there, document a bit
more about struct set_gitdir_args.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>