Amending a previous run

What happens if you run the tool and you forget to bump a package? It'd be nice if you could run the tool again and instead of creating a new branch it would just change the one you're on. (It might have to check that it was a release branch first though.)

Start by reading the command-line tool's current branch-creation behavior and how it identifies the branch being used. Define the behavior for rerunning after a missed package bump, including whether the current branch must be a release branch. Done means rerunning updates the existing release branch instead of creating a new one, with the safety condition covered.

Written by the indexing model from the issue text.