send-email: ask about and declare 8bit mails

send-email: ask about and declare 8bit mails

git-send-email passes on an 8bit mail as-is even if it does not
declare a content-type.  Because the user can edit email between
format-patch and send-email, such invalid mails are unfortunately not
very hard to come by.

Make git-send-email stop and ask about the encoding to use if it
encounters any such mail.  Also provide a configuration setting to
permanently configure an encoding.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>