Make `unsafe.force-push` visible

[ChrisPenner]

Overview

unsafe.push-force exists, but I couldn't find it when I went looking for it, figured we should probably expose it.

The name implies it's unsafe, and most folks know how a force push works anyways.

Implementation notes

• Make the command visible in help.
• Add an alias so you can find it under push.

Loose ends

I'm not sure if we support push.force-with-lease but IMO it should be the default 🤔