builtin format-patch: squelch content-type for 7-bit ASCII

When --attach is not used, usually we do not say Content-Type:
and fluff, but if the commit message is not 7-bit ASCII, mark
it as "text/plain; charset=UTF-8".  This unclutters output
somewhat.

Signed-off-by: Junio C Hamano <junkio@cox.net>