Avoid composing too long "References" header.

The number of characters in a line MUST be no more than 998 characters,
and SHOULD be no more than 78 characters (RFC2822).
It is much safer to fold the header by ourselves.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -595,7 +595,7 @@ sub send_message
 	if ($chain_reply_to || !defined $reply_to || length($reply_to) == 0) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
-			$references .= " $message_id";
+			$references .= "\n $message_id";
 		} else {
 			$references = "$message_id";
 		}