Have some way to check (+docu) if a results commit hash really exists

[AKuederle]

With each result we store the commit hash. However, it might happen that you rebase a branch, that was used to generate the results. We need to document that this must not be done.

In addition, it would be great to have some tooling that can verify, that all the commits referenced in the results actually still exist.

We should also change the repo defaults to "merge" by default.