revision: ensure MERGE_HEAD is a ref in prepare_show_merge
This is done to
(1) ensure MERGE_HEAD is a ref,
(2) obtain the oid without any prefixing by refs.c:repo_dwim_ref()
(3) error out when MERGE_HEAD is a symref.
Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>