format-patch: do not wrap non-rfc2047 headers too early

format-patch: do not wrap non-rfc2047 headers too early

Do not wrap the second and later lines of non-rfc2047-encoded headers
substantially before the 78 character limit.

Instead of passing the remaining length of the first line as wrapping
width, use the correct maximum length and tell strbuf_add_wrapped_bytes()
how many characters of the first line are already used.

Signed-off-by: Jan H. Schönherr <schnhrr@cs.tu-berlin.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>