demo git_compat and README: repurpose the demo somewhat

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.

README.md

@@ -206,8 +206,11 @@ backend. The backend exists mainly to make sure that it's possible to eventually
 add functionality that cannot easily be added to the Git backend.
 
 The Git backend is fully featured and maintained, and allows you to use Jujutsu
-as an alternative interface to Git. The commits you create will look like
-regular Git commits. You can always switch back to Git.
+with any Git remote. The commits you create will look like regular Git commits.
+You can fetch branches from a regular Git remote and push branches to the
+remote. You can always switch back to Git.
+
+Here is how you can explore a GitHub repository with `jj`.
 
 <img src="demos/git_compat.png" />
 

demos/demo_git_compat.sh

@@ -8,10 +8,26 @@ comment "Clone a Git repo:"
 run_command "jj git clone https://github.com/octocat/Hello-World"
 run_command "cd Hello-World"
 
-comment "Inspect it:"
-run_command "jj log -r 'all()'"
 blank
+
+comment "By default, \"jj\" creates a local master branch tracking the remote master
+branch. The other branches are only available as remote-tracking branches."
+run_command "jj branch list --all"
+comment "We can create a local branch tracking one of the remote branches we just
+fetched."
+run_command "jj branch track octocat-patch-1@origin"
+
+comment "By default, \"jj log\" excludes untracked remote branches to focus on
+\"our\" commits."
+run_command "jj log"
+
+comment "We can also ask \"jj\" to show all the commits."
+run_command "jj log -r 'all()'"
+
+comment "We can look at the diffs of commits in the repo"
 run_command "jj diff -r b1"
+blank
+run_command "jj diff -r b3"
 
 comment "The repo is backed by the actual Git repo:"
 run_command "git --git-dir=.jj/repo/store/git log --graph --all --decorate --oneline"