Auto-select base branch as start point for PRs in GitHub Actions

[epompeii]

Currently, users have to manually set the --branch-start-point and --branch-start-point-hash in GitHub Actions, even though all of this information is available in the GitHub Actions context. This context is what the canonical example uses for these options. So why not just do this for the user.

Now that fallible start points have been implemented: #450
This opens the door to optimistically auto-selecting the base branch as the start point for PRs.