Bisect

`git bisect` is a binary search over history. You tell it one commit where the problem exists and one where it does not; it checks out the middle and asks you which side that is on.

Each answer halves the range, so a thousand commits take ten tests. Nobody reads a thousand diffs, which is why this is the command to reach for first rather than last.

`git bisect run <command>` does it without you: any command that exits zero for good and non-zero for bad will drive the whole search.