builtin/pack-objects.c: support --max-pack-size with --cruft

When pack-objects learned the `--cruft` option back in b757353676
(builtin/pack-objects.c: --cruft without expiration, 2022-05-20), we
explicitly forbade `--cruft` with `--max-pack-size`.

At the time, there was no specific rationale given in the patch for not
supporting the `--max-pack-size` option with `--cruft`. (As best I can
remember, it's because we were trying to push users towards only ever
having a single cruft pack, but I cannot be sure).

However, `--max-pack-size` is flexible enough that it already works with
`--cruft` and can shard unreachable objects across multiple cruft packs,
creating separate ".mtimes" files as appropriate. In fact, the
`--max-pack-size` option worked with `--cruft` as far back as
b757353676!

This is because we overwrite the `written_list`, and pass down the
appropriate length, i.e. the number of objects written in each pack
shard.

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

Documentation/git-pack-objects.txt

@@ -116,9 +116,7 @@ unreachable object whose mtime is newer than the `--cruft-expiration`).
 +
 Incompatible with `--unpack-unreachable`, `--keep-unreachable`,
 `--pack-loose-unreachable`, `--stdin-packs`, as well as any other
-options which imply `--revs`. Also incompatible with `--max-pack-size`;
-when this option is set, the maximum pack size is not inferred from
-`pack.packSizeLimit`.
+options which imply `--revs`.
 
 --cruft-expiration=<approxidate>::
 	If specified, objects are eliminated from the cruft pack if they