Remove pack input from PostFetchJob

[derrickstolee]

We could use the --reachable option in git commit-graph write instead of --stdin-packs. This would remove the need to communicate a list of pack-indexes when queueing the PostFetchStep.

This will make the commit-graph smaller, but will increase the lag from when users update their refs and when the commit-graph has parsed their commits. Background prefetch can do a lot to keep them up-to-date.

[derrickstolee]

Creating for tracking. We may not actually want to do this.