fixup! test-tool: add pack-deltas helper

Junio says in https://lore.kernel.org/git/[email protected]:

> I needed this to make
>
> $ SANITIZE=leak GIT_TEST_PASSING_SANITIZE_LEAK=true make
> $ cd t && sh t5309-pack-delta-cycles.sh
>
> pass.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: gitster

t/helper/test-pack-deltas.c

@@ -122,6 +122,7 @@ int cmd__pack_deltas(int argc, const char **argv)
 			if (get_oid_hex(base_oid_str, &base_oid))
 				die("invalid object: %s", base_oid_str);
 		}
+		string_list_clear(&items, 0);
 
 		if (!strcmp(type_str, "REF_DELTA"))
 			write_ref_delta(f, &content_oid, &base_oid);