RemoteRepository.get_graph().heads(...) repeatedly queries ghosts

remote_repo.lock_write(); remote_repo.get_graph().heads(...) repeatedly queries any ghosts it encounters, because RemoteRepository's _unstacked_provider never caches missing results. Although not caching misses is good so as to be safe in the general case, for the duration of a heads() call no new keys are going to appear in the remote repository, so repeatedly querying for them is a waste of time.

This shows up in "case 4" of http://bazaar-vcs.org/SmartPushAnalysis1.13; because the divergence check — which is implemented client-side using heads() — encounters <email address hidden> and <email address hidden>, which are ghosts.