oid-array: teach oid-array to handle multiple kinds of oids

oid-array: teach oid-array to handle multiple kinds of oids

While looking at how to handle input of both SHA-1 and SHA-256 oids in
get_oid_with_context, I realized that the oid_array in
repo_for_each_abbrev might have more than one kind of oid stored in it
simultaneously.

Update to oid_array_append to ensure that oids added to an oid array
always have an algorithm set.

Update void_hashcmp to first verify two oids use the same hash algorithm
before comparing them to each other.

With that oid-array should be safe to use with different kinds of
oids simultaneously.

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