Merge branch 'km/send-email-compose-encoding'

"git send-email --compose" can let the user create a non-ascii
cover letter message, but there was not a way to mark it with
appropriate content type before sending it out.

Further updates fix subject quoting.

* km/send-email-compose-encoding:
  git-send-email: add rfc2047 quoting for "=?"
  git-send-email: introduce quote_subject()
  git-send-email: skip RFC2047 quoting for ASCII subjects
  git-send-email: use compose-encoding for Subject
  git-send-email: introduce compose-encoding