midx-write.c: extract `fill_pack_from_midx()`

midx-write.c: extract `fill_pack_from_midx()`

When filling packs from an existing MIDX, `fill_packs_from_midx()`
handles preparing a MIDX'd pack, and reading out its pack name from the
existing MIDX.

MIDX compaction will want to perform an identical operation, though the
caller will look quite different than `fill_packs_from_midx()`. To
reduce any future code duplication, extract `fill_pack_from_midx()`
from `fill_packs_from_midx()` to prepare to call our new helper function
in a future change.

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