t0021/rot13-filter: improve error message

If there is no new line at the end of something it receives,
the packet_txt_read() function die()s, but it's difficult to
debug without much context.

Let's give a bit more information when that happens.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chriscool

t/t0021/rot13-filter.pl

@@ -96,7 +96,8 @@ sub packet_bin_read {
 sub packet_txt_read {
 	my ( $res, $buf ) = packet_bin_read();
 	unless ( $res == -1 or $buf eq '' or $buf =~ s/\n$// ) {
-		die "A non-binary line MUST be terminated by an LF.";
+		die "A non-binary line MUST be terminated by an LF.\n"
+		    . "Received: '$buf'";
 	}
 	return ( $res, $buf );
 }