midx-write.c: factor out common want_included_pack() routine

When performing a 'git multi-pack-index repack', the MIDX machinery tries to aggregate MIDX'd packs together either to (a) fill the given `--batch-size` argument, or (b) combine all packs together. In either case (using the `midx-write.c::fill_included_packs_batch()` or `midx-write.c::fill_included_packs_all()` function, respectively), we evaluate whether or not we want to repack each MIDX'd pack, according to whether or it is loadable, kept, cruft, or non-empty. Between the two `fill_included_packs_` callers, they both care about the same conditions, except for `fill_included_packs_batch()` which also cares that the pack is non-empty. We could extract two functions (say, `want_included_pack()` and a `_nonempty()` variant), but this is not necessary. For the case in `fill_included_packs_all()` which does not check the pack size, we add all of the pack's objects assuming that the pack meets all other criteria. But if the pack is empty in the first place, we add all of its zero objects, so whether or not we "accept" or "reject" it in the first place is irrelevant. This change improves the readability in both `fill_included_packs_` functions. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>