midx-write.c: use `--stdin-packs` when repacking

midx-write.c: use `--stdin-packs` when repacking

When constructing a new pack `git multi-pack-index repack` provides a
list of objects which is the union of objects in all MIDX'd packs which
were "included" in the repack.

Though correct, this typically yields a poorly structured pack, since
providing the objects list over stdin does not give pack-objects a
chance to discover the namehash values for each object, leading to
sub-optimal delta selection.

We can use `--stdin-packs` instead, which has a couple of benefits:

  - it does a supplemental walk over objects in the supplied list of
    packs to discover their namehash, leading to higher-quality delta
    selection

  - it requires us to list far less data over stdin; instead of listing
    each object in the resulting pack, we need only list the
    constituent packs from which those objects were selected in the MIDX

Of course, this comes at a slight cost: though we save time on listing
packs versus objects over stdin[^1] (around ~650 milliseconds), we add a
non-trivial amount of time walking over the given objects in order to
find better deltas.

In general, this is likely to more closely match the user's expectations
(i.e. that packs generated via `git multi-pack-index repack` are written
with high-quality deltas). But if not, we can always introduce a new
option in pack-objects to disable the supplemental object walk, which
would yield a pure CPU-time savings, at the cost of the on-disk size of
the resulting pack.

[^1]: In a patched version of Git that doesn't perform the supplemental
  object walk in `pack-objects --stdin-packs`, we save around ~650ms
  (from 5.968 to 5.325 seconds) when running `git multi-pack-index
  repack --batch-size=0` on git.git with all objects packed, and all
  packs in a MIDX.

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