remote-testgit: implement the "done" feature manually

remote-testgit: implement the "done" feature manually

People who want to write their own remote-helper will find it more
useful to see clearly how they are supposed to advertise and implement
the "done" feature themselves.

Right now we are relying on fast-export to do that by using the
--use-done-feature argument. However, people writing their own
remote-helper would probably not have such an option, as they would
probably be writing the fast-export functionality themselves.

It should now be clearer to them.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>