This avoids `Wide character in print' warnings and ensures the
UTF-8 characters in `Signed-off-by' trailers are properly rendered
in HTML even when attempting to decode and display
application/octet-stream mbox attachments as HTML.
Linkification and reconstruction for coderepos is probably
still broken, but that is a much bigger task to fix, I think.
Fixes: ab9c03ff4aa369b3 ("www: use PerlIO::scalar (zfh) for buffering")