midx.c: remove unnecessary loop condition

midx.c: remove unnecessary loop condition

The fill_included_packs_batch() routine is responsible for aggregating
objects in packs with a non-zero value for the `--batch-size` option of
the `git multi-pack-index repack` sub-command.

Since this routine is explicitly called only when `--batch-size` is
non-zero, there is no point in checking that this is the case in our
loop condition.

Remove the unnecessary part of this condition to avoid confusion.

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