demo git_compat: 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. 

I also added a sentence to the README to motivate branches
appearing in the demo.

README.md

@@ -203,8 +203,9 @@ 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. The Git support uses the
-[libgit2](https://libgit2.org/) C library.
+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. The Git support uses
+the [libgit2](https://libgit2.org/) C library.
 
 
 <img src="demos/git_compat.png" />

demos/demo_git_compat.sh

@@ -8,10 +8,23 @@ comment "Clone a Git repo:"
 run_command "jj git clone https://github.com/octocat/Hello-World"
 run_command "cd Hello-World"
 
-comment "Inspect it:"
+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\" shows the commits jj considers \"ours\" together
+with their parents."
+run_command "jj log"
+
+comment "We can also ask \"jj\" to show all the commits."
 run_command "jj log -r 'all()'"
-blank
+
+comment "We can look at the commits in the repo"
 run_command "jj diff -r b1"
+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"