make repo-token optional

[davidnuescheler]

i think it would be great if the repo-token was optional as it is not immediately clear why a regular helix pages project (which lives in a public repo and has very few pushes) would require that, it may very well be that it is optional already, the documentation in the README.md suggests it is required.

[tripodsan]

all actions that want to operate on github need a repo token. this can be easily provided with secrets.GITHUB_TOKEN and is generated automatically by the github runner. see https://docs.github.com/en/free-pro-team@latest/actions/reference/authentication-in-a-workflow#about-the-github_token-secret
so the installation is very simple, as no extra token needs to be generated or registered anywhere.

However, I think in the simplest case, we could just read the list of changed files contain in the payload, and not to the diff to the master. this way, the repo token is not needed.