builtin/repack.c: avoid leaking child arguments

`git repack` invokes a handful of child processes: one to write the actual pack, and optionally ones to repack promisor objects and update the MIDX. Most of these are freed automatically by calling `start_command()` (which invokes it on error) and `finish_command()` which calls it automatically. But repack_promisor_objects() can initialize a child_process, populate its array of arguments, and then return from the function before even calling start_command(). Make sure that the prepared list of arguments is freed by calling child_process_clear() ourselves to avoid leaking memory along this path. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>