It is important to mention tracking branches and listing branches. This topic is important now that `jj log` does not show branches
`jj git clone` says it fetched by default.
The demo has become more about navigating a Git remote, so I reworded
the README accordingly, shifting the focus more to "Git remote
compatibility" and away from "We include a Git repo". Both are
important, but these skills seem important for beginners.
This updates demos to the point where they can run without
warnings. Some of them become less clear, this is addressed
(where I noticed it) in follow-up commits.
Fixes#3209
