combine-diff.c: remove implicit dependency on the_index

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1054,7 +1054,8 @@ static void show_patch_diff(struct combine_diff_path *elem, int num_parent,
 			if (is_file) {
 				struct strbuf buf = STRBUF_INIT;
 
-				if (convert_to_git(&the_index, elem->path, result, len, &buf, global_conv_flags_eol)) {
+				if (convert_to_git(rev->diffopt.repo->index,
+						   elem->path, result, len, &buf, global_conv_flags_eol)) {
 					free(result);
 					result = strbuf_detach(&buf, &len);
 					result_size = len;