format-patch: generate MIME header as needed even when there is format.header

format-patch: generate MIME header as needed even when there is format.header

Earlier, the callchain from pretty_print_commit() down to pp_title_line()
had an unwarranted assumption that the presense of "after_subject"
parameter, means the caller has already output MIME headers for
attachments.  The parameter's primary purpose is to give extra header
lines the caller wants to place after pp_title_line() generates the
"Subject: " line.

This assumption does not hold when the user used the format.header
configuration variable to pass extra headers, and caused a message with
non-ASCII character to lack proper MIME headers (e.g.  8-bit CTE header).
The earlier logic also failed to suppress duplicated MIME headers when
"format-patch -s --attach" is asked for and the signer's name demanded
8-bit clean transport.

This patch fixes the logic by introducing a separate need_8bit_cte
parameter passed down the callchain.  This can have one of these values:

 -1 : we've already done MIME crap and we do not want to add extra header
      to say this is 8bit in pp_title_line();

  0 : we haven't done MIME and we have not seen anything that is 8bit yet;

  1 : we haven't done MIME and we have seen something that is 8bit;
      pp_title_line() must add MIME header.

It adds two tests by Jeff King who independently diagnosed this issue.

Signed-off-by: Junio C Hamano <gitster@pobox.com>