builtin/repack.c: pass "out" to `prepare_pack_objects`

builtin/repack.c: pass "out" to `prepare_pack_objects`

`builtin/repack.c`'s `prepare_pack_objects()` is used to prepare a set
of arguments to a `pack-objects` process which will generate a desired
pack.

A future patch will add an `--expire-to` option which allows `git
repack` to write a cruft pack containing the pruned objects out to a
separate repository. Prepare for this by teaching that function to write
packs to an arbitrary location specified by the caller.

All existing callers of `prepare_pack_objects()` will pass `packtmp` for
`out`, retaining the existing behavior.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>