copy-tracking: Initial Implementation

[fowles]

The usage of this is not tied into the rest of jj so I have not updated the changelog, but I would like to move forward with this.

Checklist

If applicable:

• [ ] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [x] I have added tests to cover my changes