Verify the Git branch is up-to-date

[hugovk]

To prevent releasing using an old branch: python/cpython#143838.

Let's add a check near the start to ensure the branch is up-to-date.

This should be a yes/no confirmation warning, rather than a hard failure, because we might build a security branch that hasn't had commits for some time.

At the same time, we can add a check that repo is on the correct initial branch (main, 3.14, ...).

We could also consider automating the git fetch/pull.