midx-write.c: extract inner loop from fill_packs_from_midx()

midx-write.c: extract inner loop from fill_packs_from_midx()

The function fill_packs_from_midx() does relatively little, but ends up
in a doubly-nested loop because we're enumerating each pack within each
layer of the incremental MIDX chain.

Let's de-dent the inner loop of fill_packs_from_midx() by extracting its
contents into a separate function, and calling that.

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