git-submodule.sh: get rid of unused variable

Remove the variable "$diff_cmd" which is no longer used.

Signed-off-by: Roy Eldar <royeldar0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Roy Eldar

git-submodule.sh

@@ -503,7 +503,6 @@ cmd_set_url() {
 cmd_summary() {
 	summary_limit=-1
 	for_status=
-	diff_cmd=diff-index
 
 	# parse $args after "submodule ... summary".
 	while test $# -ne 0