get_helper: use run-command's internal argv_array

get_helper: use run-command's internal argv_array

The get_helper functions dynamically allocates an
argv_array, feeds it to start_command, and then returns. We
then have to later clean up the memory manually after
calling finish_command. We can make this simpler by just
using run-command's internal argv_array, which handles
cleanup for us.

This also prevents a memory leak in the case that
transport_take_over is used, in which case we free the child
in finish_connect, which does not manually free the array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>