git chain push should set upstream (remote) automatically if one does not exist. This is more smarter than bailing out naively:

Do not do this for git chain push --force since it's a dangerous operation.

Bonus: if git chain push --force fails, then suggest that the user run git chain push if upstream (remote) branches are detected to not be set.

Starting points:

• https://docs.rs/git2/latest/git2/struct.Repository.html#method.remotes
• https://docs.rs/git2/0.13.25/git2/struct.Branch.html#method.set_upstream
• https://docs.rs/git2/0.13.25/git2/struct.Branch.html#method.set_upstream