submodule: fix leaking update strategy

We're not freeing the submodule update strategy command. Provide a
helper function that does this for us and call it in
`update_data_release()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -424,6 +424,11 @@ int parse_submodule_update_strategy(const char *value,
 	return 0;
 }
 
+void submodule_update_strategy_release(struct submodule_update_strategy *strategy)
+{
+	free((char *) strategy->command);
+}
+
 const char *submodule_update_type_to_string(enum submodule_update_type type)
 {
 	switch (type) {