t5703: use object-format serve option

t5703: use object-format serve option

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 wrapper function that specifies this header if required.
Skip specifying this header for SHA-1 to test that it works both with an
without this header.

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