manifestcache: make sure the entry are ordered by access time This is an LRU cache, let us make sure of that.

manifestcache: adding a second distinct entry Let makes sure the cache can hold multiple value.

manifestcache: test that adding the same entry twice do not duplicates it Simple sanity check.

manifestcache: do not display data when using --add If the command invocation is about adding a new entry, we should remain terse (the same as we do for many commands).