t/helper: fix leaking buffer in "dump-untracked-cache"

We never release the local `struct strbuf base` buffer, thus leaking
memory. Fix this leak.

This leak is exposed by t7063, but plugging it alone does not make the
whole test suite pass.

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

t/helper/test-dump-untracked-cache.c

@@ -68,5 +68,7 @@ int cmd__dump_untracked_cache(int ac UNUSED, const char **av UNUSED)
 	printf("flags %08x\n", uc->dir_flags);
 	if (uc->root)
 		dump(uc->root, &base);
+
+	strbuf_release(&base);
 	return 0;
 }