contrib: add a commit synthesizer for reproducing scaling problems This adds two new commands: - analyze examines an existing repo and writes out a statistical description of its properties that contains no identifying information. - synthesize creates new commits based on the description generated by analyze. The intention is that a repo constructed using synthesize will have properties that are vaguely statistically similar to the originating repo, but entirely random content. This can be useful for forecasting performance as a repo grows, and for developers who want to find bottlenecks in proprietary repos to which they do not have access.