Guard rails against pushing empty branches

[maritz]

Several times now I have accidentally pushed branches without commiting any changes. When doing gs branch create without any files staged, it creates an empty commit that will then be pushed when doing gs branch submit after.

This is of course a user error and based on it being Monday morning.

However maybe the tooling could be made aware of my dumbness and prevent me from pushing branches that don't have any actual changes in them.

[abhinav]

This makes sense. We can add a confirmation prompt (possibly with an opt out config) for submit if the branch is empty. we already have similar prompts for amending on main and other "you might not want that" interactions.

Confirmation should default to true so that user can press enter a few times to submit instead of Y then enter.

[abhinav]

tracking in #878