cache: make oidcpy always copy GIT_MAX_RAWSZ bytes

cache: make oidcpy always copy GIT_MAX_RAWSZ bytes

There are some situations in which we want to store an object ID into
struct object_id without the_hash_algo necessarily being set correctly.
One such case is when cloning a repository, where we must read refs from
the remote side without having a repository from which to read the
preferred algorithm.

In this cases, we may have the_hash_algo set to SHA-1, which is the
default, but read refs into struct object_id that are SHA-256. When
copying these values, we will want to copy them completely, not just the
first 20 bytes. Consequently, make sure that oidcpy copies the maximum
number of bytes at all times, regardless of the setting of
the_hash_algo.

Since oidcpy and hashcpy are no longer functionally identical, remove
the Cocinelle object_id transformations that convert from one into the
other.

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