If we already have a strbuf, then using write_file_buf is a
little nicer to read (no wondering whether "%s" will eat
your NULs), and it's more efficient (no extra formatting
step).
We don't care about the newline magic of write_file(), as we
have our own multi-line content.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>