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

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