remote-testgit: adapt for object-format

remote-testgit: adapt for object-format

When using an algorithm other than SHA-1, we need the remote helper to
advertise support for the object-format extension and provide
information back to us so that we can properly parse refs and return
data. Ensure that the test remote helper understands these extensions.

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