repo sync: update all tracked branches

Right now, `repo sync` is incorrect.
The name suggests it updates the entire repository, but what it does is just update main.
What it should do:

- pull latest changes for all tracked branches and main
- delete all merged branches
- restack whatever was affected

For the case currently handled by `repo sync`—single branch sync, we'll need to add a `branch sync` command.
We may also add stack-scope sync commands.