lib: Add an API to list only "our" objects, fix prune to use it

When doing a prune, we should not try to delete objects in parent
repos, since it'll fail.  There is a bigger discussion about the
semantics of `parent=` to be had, but this will fix trying to use
`ostree prune --repo=/ostree/repo/extensions/rpmostree/pkgcache`.

Closes: https://github.com/ostreedev/ostree/issues/467

Closes: #471
Approved by: jlebon

src/libostree/ostree-repo-prune.c

@@ -337,8 +337,8 @@ ostree_repo_prune (OstreeRepo        *self,
         }
     }
 
-  if (!ostree_repo_list_objects (self, OSTREE_REPO_LIST_OBJECTS_ALL, &objects,
-                                 cancellable, error))
+  if (!ostree_repo_list_objects (self, OSTREE_REPO_LIST_OBJECTS_ALL | OSTREE_REPO_LIST_OBJECTS_NO_PARENTS,
+                                 &objects, cancellable, error))
     goto out;
 
   if (!refs_only)