When we write a tag the object oid is specific to the hash algorithm. This matters when a tag is signed. The hash transition plan calls for signatures on both the sha1 form and the sha256 form of the object, and for both of those signatures to live in the tag object. To generate tag object with multiple signatures, first compute the unsigned form of the tag, and then if the tag is being signed compute the unsigned form of the tag with the compatibilityr hash. Then compute compute the signatures of both buffers. Once the signatures are computed add them to both buffers. This allows computing the compatibility hash in do_sign, saving write_object_file the expense of recomputing the compatibility tag just to compute it's hash. Signed-off-by: Eric W. Biederman <ebiederm@xmission.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>