Improve Github Public Hashes

[tja]

As implemented in #4, we fetch the event stream from the GH Archive project and parse events of type PushEvent to get SHA commit hashes. There are two issues with this approach:

1. As described in GitHub's documentation, a PushEvent contains a maximum of 20 commits. It is recommended to fetch additional commits using the CommitsAPI.

2. Repositories created privately and then made public are simply ignored, because there is no PushEvent involved, just a PublicEvent with no payload.

We should improve our current GH archive parser to detect a PushEvent with more than 20 commits, as well as understand the PublicEvent type. In both cases, the Commit API should be used to retrieve all public commits.