cache: add a function to read an OID of a specific algorithm

cache: add a function to read an OID of a specific algorithm

Currently, we always read a object ID of the current algorithm with
oidread.  However, once we start converting objects, we'll need to
consider what happens when we want to read an object ID of a specific
algorithm, such as the compatibility algorithm.  To make this easier,
let's define oidread_algop, which specifies which algorithm we should
use for our object ID, and define oidread in terms of it.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>