Merge branch 'jc/checkdiff'

Merge branch 'jc/checkdiff'

* jc/checkdiff:
  Fix t4017-diff-retval for white-space from wc
  Update sample pre-commit hook to use "diff --check"
  diff --check: detect leftover conflict markers
  Teach "diff --check" about new blank lines at end
  checkdiff: pass diff_options to the callback
  check_and_emit_line(): rename and refactor
  diff --check: explain why we do not care whether old side is binary