midx.c: avoid cruft packs with non-zero `repack --batch-size`

midx.c: avoid cruft packs with non-zero `repack --batch-size`

Apply similar treatment with respect to cruft packs as in a few commits
ago to `repack` with a non-zero `--batch-size`.

Since the case of a non-zero `--batch-size` is handled separately (in
`fill_included_packs_batch()` instead of `fill_included_packs_all()`), a
separate fix must be applied for this case.

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