Scale/performance test the bundle server

[vdye]

If we want the bundle server to support very large Git repositories (which we do), we need to make sure it can accommodate repositories that large. To do that, we should run (either manual or automated) scale tests to see how the bundle server performs w.r.t updates, specifically:

• What happens if it's collapsing bundles while serving old bundles?
• How slow is it to collapse a bundle list? Is there a way to do it faster?
• What happens if git-bundle-server update is invoked while the previous update is in-progress?

We can use the answers to those questions to make changes as needed to support large repositories.