repo sync: update all tracked branches

[abhinav]

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.