midx: switch to using the_hash_algo

midx: switch to using the_hash_algo

Instead of hard-coding the hash size, use the_hash_algo to look up the
hash size at runtime.  Remove the #define constant which was used to
hold the hash length, since writing the expression with the_hash_algo
provide enough documentary value on its own.

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