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.