what about just doing a git diff ? that would see the method was not called before?

If it were a longstanding bug versus one just created for this exercise a git diff may not help much. Imagine you've found some edge case in your code that just hadn't been properly exercised before. It could have been there for years, now how do you isolate the erroring section? This technique (or the one I mentioned in my other comment which is very similar but uses more data) can help isolate the problem section.

git diffs can definitely help with newer bugs or if you can show that it's a regression (didn't error before commit 1234abcd but did after).