builtin/repack.c: avoid leaking child arguments

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>