Fix the repo name for testing the Travis tagger

(See both commits separately)

The travis auto tagger pushes a release tag to the repository when the
build (on master) is successful.

The script allows you to test it in local mode. To do so, you need to fake
the repo name (which would normally come from the Travis environment)

This fixes the repo name to be `alphagov/performanceplatform-collector`
rather than `alphagov/stagecraft` and adds a testing commit from the repo,
rather than from the stagecraft repo.

Also move function to more obvious place so this is less likely to happen again.