Git Bisect: Find the commit that broke my tests

So master is broken and you don't know which commit contains the offending code....

Git bisect is an amazing tool. It allows you to flag which commit was the last green state, and automatically iterate over each subsequent commit, running a script (i.e. your tests) until it finds the breaking commit....

i.e.

git bisect start [some-breaking-commit-sha] [last-good-commit-sha]
git bisect run [test-command]

a working example using rspec:

git bisect start 120cba5 c2d0a5e
git bisect run rspec spec/mytest_spec:123