send-email: fix nasty bug in ask() function

Commit 6e182518 (send-email: refactor and ensure prompting doesn't loop
forever) introduced an ask function, which unfortunately had a nasty
bug. This caused it not to accept anything but the default reply to the
"Who should the emails appear to be from?" prompt, and nothing but
ctrl-d to the "Who should the emails be sent to?" and "Message-ID to be
used as In-Reply-To for the first email?" prompts.

This commit corrects the issues and adds a test to confirm the fix.
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -608,7 +608,7 @@ ($)
 
 sub ask {
 	my ($prompt, %arg) = @_;
-	my $valid_re = $arg{valid_re} || ""; # "" matches anything
+	my $valid_re = $arg{valid_re};
 	my $default = $arg{default};
 	my $resp;
 	my $i = 0;
@@ -624,7 +624,7 @@ sub ask {
 		if ($resp eq '' and defined $default) {
 			return $default;
 		}
-		if ($resp =~ /$valid_re/) {
+		if (!defined $valid_re or $resp =~ /$valid_re/) {
 			return $resp;
 		}
 	}

t/t9001-send-email.sh

@@ -130,6 +130,19 @@ test_expect_success 'Show all headers' '
 	test_cmp expected-show-all-headers actual-show-all-headers
 '
 
+test_expect_success 'Prompting works' '
+	clean_fake_sendmail &&
+	(echo "Example <from@example.com>"
+	 echo "to@example.com"
+	 echo ""
+	) | GIT_SEND_EMAIL_NOTTY=1 git send-email \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		$patches \
+		2>errors &&
+		grep "^From: Example <from@example.com>$" msgtxt1 &&
+		grep "^To: to@example.com$" msgtxt1
+'
+
 z8=zzzzzzzz
 z64=$z8$z8$z8$z8$z8$z8$z8$z8
 z512=$z64$z64$z64$z64$z64$z64$z64$z64