When we run a dashed external, we use the one-liner
run_command_v_opt() to do so. Let's switch to using a
child_process struct, which has two advantages:
1. We can drop all of the allocation and cleanup code for
building our custom argv array, and just rely on the
builtin argv_array (at the minor cost of doing a few
extra mallocs).
2. We have access to the complete range of child_process
options, not just the ones that the "_opt()" form can
forward.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>