test-pkt-line: drop colon from sideband identity

We pass "sideband: " as our identity for errors to recv_sideband(). But
it already adds the trailing colon and space. This doesn't invalidate
any tests, but it looks funny when you examine the test output.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-pkt-line.c

@@ -100,7 +100,7 @@ static int send_split_sideband(void)
 
 static int receive_sideband(void)
 {
-	return recv_sideband("sideband: ", 0, 1);
+	return recv_sideband("sideband", 0, 1);
 }
 
 int cmd__pkt_line(int argc, const char **argv)