Use git-merge-base for getting the golangci-lint start commit
Use git-merge-base for getting the golangci-lint start commit

The current static approach yields too many false negatives (i.e. old
lint errors we're deciding to ignore for now) in certain scenarios.
I've tried git-merge-base for this with mixed results before, but it
might be an improvement over the current situation.

loadimpact/k6,loadimpact/k6