format-patch: simplify after-subject MIME header handling
In log_write_email_headers(), we append our MIME headers to the set of
extra headers by creating a new strbuf, adding the existing headers, and
then adding our new ones. We had to do it this way when our output
buffer might point to the constant opt->extra_headers variable.
But since the previous commit, we always make a local copy of that
variable. Let's turn that into a strbuf, which lets the MIME code simply
append to it. That simplifies the function and avoids a pointless extra
copy of the headers.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>