list_objects_filter_copy(): deep-copy sparse_oid_name field

The purpose of our copy function is to do a deep copy of each field so
that the source and destination structs become independent. We correctly
copy the filter_spec string list, but we forgot the sparse_oid_name
field. By doing a shallow copy of the pointer, that puts us at risk for
a use-after-free if one or both of the structs is cleaned up.

I don't think this can be triggered in practice, because we tend to leak
the structs rather than actually clean them up. But this should
future-proof us for plugging those leaks.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter-options.c

@@ -418,6 +418,7 @@ void list_objects_filter_copy(
 	string_list_init_dup(&dest->filter_spec);
 	for_each_string_list_item(item, &src->filter_spec)
 		string_list_append(&dest->filter_spec, item->string);
+	dest->sparse_oid_name = xstrdup_or_null(src->sparse_oid_name);
 
 	ALLOC_ARRAY(dest->sub, dest->sub_alloc);
 	for (i = 0; i < src->sub_nr; i++)