ci_run_n_monitor: warn user if they forgot to push the branch
Only perform this check if they set --rev HEAD (or don't specify it);
let's assume if they select another commit, they know what they're
doing.
I've had several users ask me why it took so long to for the pipeline to appear, and it turns out they assumed the ci_run_n_monitor script would push their commit somewhere for them.
Let's warn them that they need to git push the commit they want to test.