Give git-am back the ability to add Signed-off-by lines.

This was lost in the migration to git-rev-parse --parseopt by commit
78443d9049.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -131,7 +131,7 @@ do
 		binary=t ;;
 	-3|--3way)
 		threeway=t ;;
-	-s--signoff)
+	-s|--signoff)
 		sign=t ;;
 	-u|--utf8)
 		utf8=t ;; # this is now default