builtin/repack: fix leaking configuration

builtin/repack: fix leaking configuration

When repacking, we assemble git-pack-objects(1) arguments both for the
"normal" pack and for the cruft pack. This configuration gets populated
with a bunch of `OPT_PASSTHRU` options that we end up passing to the
child process. These options are allocated, but never free'd.

Create a new `pack_objects_args_release()` function that releases the
memory for us and call it for both sets of options.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>