archive: convert struct archiver_args to object_id

Change the commit_sha1 member to be called "commit_oid" and change it to
be a pointer to struct object_id.  Additionally, update some uses of
GIT_SHA1_HEXSZ and hard-coded values to use the_hash_algo instead.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>