Improve live feedback messages

[jescalada]

As of #1637, users can see the step currently being executed when running git push, which isn't very valuable in itself:

git-proxy-sideband-streaming-demo.mp4

We could extend this to print out some of what we're logging with step.log, though that makes me wonder just how verbose we want the live feedback to be.

• Only basic logging for step name + errors on failed steps?
• Optional verbose flag for showing more detailed progress?

Describe the solution you'd like
Logs that are more useful than what's been done in #1637.