git-send-email: Do not attempt to STARTTLS more than once

With the previous TLS patch, send-email would attempt to STARTTLS at
the beginning of every mail, despite reusing the last connection.  We
simply skip further encryption checks after successful TLS initiation.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -772,6 +772,7 @@ X-Mailer: git-send-email $gitversion
 				if ($smtp->code == 220) {
 					$smtp = Net::SMTP::SSL->start_SSL($smtp)
 						or die "STARTTLS failed! ".$smtp->message;
+					$smtp_encryption = '';
 				} else {
 					die "Server does not support STARTTLS! ".$smtp->message;
 				}