send-email: don't attempt to prompt if tty is closed

Attempting to prompt when the tty is closed (typically when running from
cron) is pointless and emits a warning. This patch causes ask() to
return early, squelching the warning.
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -612,6 +612,9 @@ sub ask {
 	my $default = $arg{default};
 	my $resp;
 	my $i = 0;
+	return defined $default ? $default : undef
+		unless defined $term->IN and defined fileno($term->IN) and
+		       defined $term->OUT and defined fileno($term->OUT);
 	while ($i++ < 10) {
 		$resp = $term->readline($prompt);
 		if (!defined $resp) { # EOF