Merge branch 'rs/diff-exit-code-with-external-diff'

"git diff --exit-code --ext-diff" learned to take the exit status of the external diff driver into account when deciding the exit status of the overall "git diff" invocation when configured to do so. * rs/diff-exit-code-with-external-diff: diff: let external diffs report that changes are uninteresting userdiff: add and use struct external_diff t4020: test exit code with external diffs