ICSHWI-7179: Update e3 exporter

This MR primarily updates the Prometheus exporter that previously was called the e3-exporter - now to be called the git-state-exporter. It also changes how git diff is determined (to work also with checked out tags), and adds some tests.

For whatever reason, this MR has duplicates (or triplicates?) of all the commits on top of a lot of "gitlab runner-based troubleshooting," so the commits will be squashed.