Re: [PATCH] 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 it populated
> via for_each_object_in_pack(). Now we're 3% faster without the "cat"
> instead.
>
> The tests were done on an unloaded Linux 3.10 system with 10 runs for
> each.
>
> 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)