Merge branch 'jc/diff-exit-code-with-w-fixes' into maint-2.42

Merge branch 'jc/diff-exit-code-with-w-fixes' into maint-2.42

"git diff -w --exit-code" with various options did not work
correctly, which is being addressed.

* jc/diff-exit-code-with-w-fixes:
  diff: the -w option breaks --exit-code for --raw and other output modes
  t4040: remove test that succeeded for a wrong reason
  diff: teach "--stat -w --exit-code" to notice differences
  diff: mode-only change should be noticed by "--patch -w --exit-code"
  diff: move the fallback "--exit-code" code down