That's explained at the end (Revisions). Fowler is probably looking for a general term that can be used to describe this specific way of debugging. Since he is aware of git bisect (and I'm sure he knows about hg bisect) there must be a reason he is not preferring "bisect debugging," for example.

Edit: The term diff has a clear link with version control. bisect is not that obvious. It may be ambiguous/vague in debugging context. I would still call it "bisect debugging."

Because ultimately you look at the diff of changes, which significantly limits the amount of code you need to examine to find the bug (or at least something that uncovered the bug).