Merge branch 'bc/send-email-auto-cte'

The content-transfer-encoding of the message "git send-email" sends
out by default was 8bit, which can cause trouble when there is an
overlong line to bust RFC 5322/2822 limit.  A new option 'auto' to
automatically switch to quoted-printable when there is such a line
in the payload has been introduced and is made the default.

* bc/send-email-auto-cte:
  docs: correct RFC specifying email line length
  send-email: automatically determine transfer-encoding
  send-email: accept long lines with suitable transfer encoding
  send-email: add an auto option for transfer encoding