send-email: make_message_id use "require" instead of "use"

Change the use of Sys::Hostname from a "use" to a "require". The
former happens in an implicit BEGIN block and is thus immune from the
if block it's contained in, so it's always loaded.

This should speed up the invocation of git-send-email by a few
milliseconds.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.comReviewed-by: Avery Pennarun <apenwarr@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -819,7 +819,7 @@ sub make_message_id {
 		last if (defined $du_part and $du_part ne '');
 	}
 	if (not defined $du_part or $du_part eq '') {
-		use Sys::Hostname qw();
+		require Sys::Hostname;
 		$du_part = 'user@' . Sys::Hostname::hostname();
 	}
 	my $message_id_template = "<%s-git-send-email-%s>";