If the compose message contains non-ascii characters, then
we assume it is in utf-8 and include the appropriate MIME
headers. If the user has already included a MIME-Version
header, then we assume they know what they are doing and
don't add any headers.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>