core: Do enumerate content objects in archive-z2 repositories

Prune has worked fine on bare repositories for some time, but now that
I finally try to delete data on the server side, I notice we weren't
actually enumerating content objects =/

That caused them to not be pruned.

https://bugzilla.gnome.org/show_bug.cgi?id=733458
core: Do enumerate ff/ objects as well

An embarassing off-by-one here.  I noticed we weren't pruning them.

https://bugzilla.gnome.org/show_bug.cgi?id=733458
repofile: Avoid segfault if querying child in non-directory

The user might "ostree ls /usr/bin/bash/blah", which previously would
segfault.

A somewhat related future enhancement here would be for "ostree ls" to
follow symbolic links.

Reported-by: Dusty Mabe <dustymabe@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=733476