Merge branch 'ag/send-email-hostname-f'

Teach "git send-email" to also consult `hostname -f` for mail
domain to compute the identity given to SMTP servers.

* ag/send-email-hostname-f:
  send-email: try to get fqdn by running hostname -f on Linux and macOS

Author: gitster

git-send-email.perl

@@ -1393,8 +1393,22 @@ sub maildomain_mta {
 	return $maildomain;
 }
 
+sub maildomain_hostname_command {
+	my $maildomain;
+
+	if ($^O eq 'linux' || $^O eq 'darwin') {
+		my $domain = `(hostname -f) 2>/dev/null`;
+		if (!$?) {
+			chomp($domain);
+			$maildomain = $domain if valid_fqdn($domain);
+		}
+	}
+	return $maildomain;
+}
+
 sub maildomain {
-	return maildomain_net() || maildomain_mta() || 'localhost.localdomain';
+	return maildomain_net() || maildomain_mta() ||
+		maildomain_hostname_command || 'localhost.localdomain';
 }
 
 sub smtp_host_string {