transport-helper: use the new done feature where possible

transport-helper: use the new done feature where possible

In other words, use fast-export --use-done-feature to add a 'done'
command at the end of streams passed to remote helpers' "import"
commands, and teach the remote helpers implementing "export" to use
the 'done' command in turn when producing their streams.

The trailing \n in the protocol signals the helper that the
connection is about to close, allowing it to do whatever cleanup
neccesary.

Previously, the connection would already be closed by the
time the trailing \n was to be written. Now that the remote-helper
protocol uses the new done command in its fast-import streams, this
is no longer the case and we can safely write the trailing \n.

Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>