builtin/cat-file: let the oid determine the output algorithm

builtin/cat-file: let the oid determine the output algorithm

Use GET_OID_HASH_ANY when calling get_oid_with_context.  This
implements the semi-obvious behaviour that specifying a sha1 oid shows
the output for a sha1 encoded object, and specifying a sha256 oid
shows the output for a sha256 encoded object.

This is useful for testing the the conversion of an object to an
equivalent object encoded with a different hash function.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>