PackfileMaintenanceStep should dynamically resize the batch size

The PackfileMaintenanceStep expires prefetch packs where every object appears in a new pack. It then condenses many packs into one pack using git multi-pack-index repack --batch-size=<size> where "size" is currently "2gb".

We cannot make "2gb" much larger due to issues with unsigned long on Windows. This also limits the amount of work we would do in one step. With this batch size, we anticipate the users with ~100 GB of prefetch data will get to a steady state in 30-60 runs of the step.

Once in a steady state, we anticipate that it will take a long time before the repack command has enough data for a full batch of data.

We can predict if the batch size is too large by examining the pack directory and counting the number of files smaller than 2gb. We could even predict a size that would group all packs into a single pack by adding their sizes and subtracting 1. If this size is larger than 2 * 1024 * 1024 * 1024, then we can stick with the default.

Start by locating PackfileMaintenanceStep and its use of git multi-pack-index repack --batch-size=2gb. Read how the pack directory is examined and validate the proposed size calculation, including the Windows unsigned-long limit. Done when the step dynamically selects a batch size for smaller packs, retains the default when the calculated size exceeds 2 * 1024 * 1024 * 1024, and regression coverage verifies the behavior.