git bisect — binary search for regressions

When something broke "sometime in the last 200 commits," bisect finds the culprit in about 8 steps.

git bisect start
git bisect bad            # current is broken
git bisect good v1.4.0    # this tag worked
# test, then mark each checkout good/bad until git names the commit

Bonus: git bisect run ./test.sh automates the whole thing while you get coffee.