fix shell syntax error in template

An if clause must not be empty; add a "colon" command.

Signed-off-by: Thorsten Glaser <t.glaser@tarent.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-push.sample

@@ -30,6 +30,7 @@ do
 	if [ "$local_sha" = $z40 ]
 	then
 		# Handle delete
+		:
 	else
 		if [ "$remote_sha" = $z40 ]
 		then