Merge branch 'wc/diff'

* wc/diff:
  Test interaction between diff --check and --exit-code
  Use shorter error messages for whitespace problems
  Add tests for "git diff --check" with core.whitespace options
  Make "diff --check" output match "git apply"
  Unify whitespace checking
  diff --check: minor fixups
  "diff --check" should affect exit status