Divergent history in Git

[loleg]

When a developer submits a Pull Request on GitHub, many commits are visible in the log that are not part of the actual PR. These are changes that are already part of the main or preprod (i.e. staging) branch. This affects our ability to review PRs, because also many extra changes are in the diff.

Investigate if this is a known issue, see if we can get support from GitHub.

See #509 for an example.