Simplify set-changed-files.sh and make it more robust

For MR pipelines, we always have the necessary information about which commits to diff in form of their hashes already, so we just use it. This has the advantage that any changes to the branch between triggering the pipeline and running the script will not affect us.

Furthermore, there is no reason to fiddle with the git remotes. The "origin" remote always exists and it always contains the necessary commits, because in all situations GitLab creates a checkout with an origin remote and all commits that we need for the diff (in MR pipelines) are always ancestors of HEAD and thus present in this remote. It doesn't even matter whether this remote points to a fork or the upstream repo.