[feature request] Only prompt for combined PR name if we actually need to create or update it

[greglockwood]

Currently, every time you use --create-prs with a combined branch in the train, it asks before even checking if it is necessary what the combined branch PR should be called in GitHub.

This prompts some anxiety for me because I don't know if it will be used or not, so I have to go back and check what I already called it and type that out again.

It would be great if we could defer prompting the user for input until it is necessary.

For bonus points, we could try to infer a sensible name as the default value for the prompt.