Merge branch 'tr/maint-format-patch-empty-output'

* tr/maint-format-patch-empty-output:
  Document negated forms of format-patch --to --cc --add-headers
  t4014: "no-add-headers" is actually called "no-add-header"
  t4014: invoke format-patch with --stdout where intended
  t4014: check for empty files from git format-patch --stdout