git log -S <the-string>
git log --grep <the-word-or-phrase>
git blame —- $(git grep -P -l 'the.*regex') | grep -P 'the.*regex'
git log -L :<the-function-name>:<the-file-name>
But, to merge we need to have all tests pass. (If tests flakily pass then we get new flakey tests, yay!)
I know git-bisect doesn’t support this: but could git-bayesect have an option to only consider merge commits? Being able to track a flake change back to an individual PR would be really useful.