[$] Git evolve: tracking changes to changes

The Git source-code management system exists to track changes to a set offiles; the stream of commits in a Git repositoryreflects the change history of those files. What is seen in Git, though, is thefinal form of those commits; the changes that the patches themselves wentthrough on their way toward acceptance are not shown there. That historycan have value, especially while changes are still under consideration.The proposed gitevolve subcommand is a recognition that changes themselves gothrough changes and that this process might benefit from tooling support.