refs.c: use is_dir_sep() in resolve_gitlink_ref()

The "submodule" argument in this function is a path, which can have
either '/' or '\\' as a separator. Use is_dir_sep() to support both.

Noticed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>