packfile: express constants in terms of the_hash_algo

packfile: express constants in terms of the_hash_algo

Replace uses of GIT_SHA1_RAWSZ with references to the_hash_algo to avoid
dependence on a particular hash length.

It's likely that in the future, we'll update the pack format to indicate
what hash algorithm it uses, and then this code will change.  However,
at least on an interim basis, make it easier to develop on a pure
SHA-256 Git by using the_hash_algo here.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>