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.c

@@ -3590,7 +3590,7 @@ ostree_repo_list_objects (OstreeRepo                  *self,
     {
       if (!list_loose_objects (self, ret_objects, NULL, cancellable, error))
         goto out;
-      if (self->parent_repo)
+      if ((flags & OSTREE_REPO_LIST_OBJECTS_NO_PARENTS) == 0 && self->parent_repo)
         {
           if (!list_loose_objects (self->parent_repo, ret_objects, NULL, cancellable, error))
             goto out;