cache: update object ID functions for the_hash_algo

cache: update object ID functions for the_hash_algo

Most of our code has been converted to use struct object_id for object
IDs.  However, there are some places that still have not, and there are
a variety of places that compare equivalently sized hashes that are not
object IDs.  All of these hashes are artifacts of the internal hash
algorithm in use, and when we switch to NewHash for object storage, all
of these uses will also switch.

Update the hashcpy, hashclr, and hashcmp functions to use the_hash_algo,
since they are used in a variety of places to copy and manipulate
buffers that need to move data into or out of struct object_id.  This
has the effect of making the corresponding oid* functions use
the_hash_algo as well.

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