dup() the input fd for fast-import used for remote helpers

When a remote helper exposes the "import" capability, stdout of the helper is sent to stdin of a new fast-import process. This is done by setting the corresponding child_process's in field to the value of the out field of the helper child_process. The child_process API is defined to close the file descriptors it's given when calling start_command. This means when start_command is called for the fast-import process, its input fd (the output fd of the helper), is closed. But when the transport helper is later destroyed, in disconnect_helper, its input and output are closed, which means close() is called with an invalid fd (since it was already closed as per above). Or worse, with a valid fd owned by something else (since fd numbers can be reused). Signed-off-by: Mike Hommey <mh@glandium.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>