serve: advertise object-format capability for protocol v2

serve: advertise object-format capability for protocol v2

In order to communicate the protocol supported by the server side, add
support for advertising the object-format capability.  We check that the
client side sends us an identical algorithm if it sends us its own
object-format capability, and assume it speaks SHA-1 if not.

In the test, when we're using an algorithm other than SHA-1, we need to
specify the algorithm in use so we don't get a failure with an "unknown
format" message.  Add a test that we handle a mismatched algorithm.
Remove the test_oid_init call since it's no longer necessary.

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