midx.c: avoid cruft packs with `repack --batch-size=0`

midx.c: avoid cruft packs with `repack --batch-size=0`

The `repack` sub-command of the `git multi-pack-index` builtin creates a
new pack aggregating smaller packs contained in the MIDX up to some
given `--batch-size`.

When `--batch-size=0`, this instructs the MIDX builtin to repack
everything contained in the MIDX into a single pack.

In similar spirit as a previous commit, it is undesirable to repack the
contents of a cruft pack in this step. Teach `repack` to ignore any
cruft pack(s) when `--batch-size=0` for the same reason(s).

(The case of a non-zero `--batch-size` will be handled in a subsequent
commit).

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