Jujutsu is a new version control system which is rapidly gaining momentum. It works with Git remotes (and wraps Git internally for object storage, at least for now). This change adds an RBTools client for Jujutsu. Testing Done: - Ran unit tests. - Posted a variety of changes with various revision ranges. - Posted changes that included binary files. - Tested `rbt stamp` with both the head and non-head changes. - Tested `rbt land` with both merge and squash modes, and `--push`. - Tested `rbt patch` with a variety of changes, including both per-commit and squash modes. - Built docs and looked at the updated/added pages.