commit-graph write: use pack order when finding commits

Slightly optimize the "commit-graph write" step by using
FOR_EACH_OBJECT_PACK_ORDER with for_each_object_in_pack(). See commit
[1] and [2] for the facility and a similar optimization for "cat-file".

On Linux it is around 5% slower to run:

    echo 1 >/proc/sys/vm/drop_caches &&
    cat .git/objects/pack/* >/dev/null &&
    git cat-file --batch-all-objects --batch-check --unordered

Than the same thing with the "cat" omitted. This is as expected, since
we're iterating in pack order and the "cat" is extra work.

Before this change the opposite was true of "commit-graph write". We
were 6% faster if we first ran "cat" to efficiently populate the FS
cache for our sole big pack on linux.git, than if we had populated it
via for_each_object_in_pack(). Now we're 3% faster without the "cat"
instead.

My tests were done on an unloaded Linux 3.10 system with 10 runs for
each. Derrick Stolee did his own tests on Windows[3] showing a 2%
improvement with a high degree of accuracy.

1. 736eb88fdc ("for_each_packed_object: support iterating in
   pack-order", 2018-08-10)

2. 0750bb5b51 ("cat-file: support "unordered" output for
   --batch-all-objects", 2018-08-10)

3. https://public-inbox.org/git/f71fa868-25e8-a9c9-46a6-611b987f1a8f@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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