In a later patch we want to determine if a path is suitable as an
alternate from other commands than builtin/clone. Move the checking
functionality of `add_one_reference` to `compute_alternate_path` that is
defined in cache.h.
Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>