It does not make sense to look outside refs/heads for HEAD's target
(src_ref_prefix can be set to "refs/" if --mirror is used) because ref
code only allows symref in form refs/heads/...
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>