Emit the notes attached to the commit in "format-patch --notes"
output after three-dashes.

* jc/prettier-pretty-note:
  format-patch: add a blank line between notes and diffstat
  Doc User-Manual: Patch cover letter, three dashes, and --notes
  Doc format-patch: clarify --notes use case
  Doc notes: Include the format-patch --notes option
  Doc SubmittingPatches: Mention --notes option after "cover letter"
  Documentation: decribe format-patch --notes
  format-patch --notes: show notes after three-dashes
  format-patch: append --signature after notes
  pretty_print_commit(): do not append notes message
  pretty: prepare notes message at a centralized place
  format_note(): simplify API
  pretty: remove reencode_commit_message()