mtree: Load traversed subdirs when creating parents

mtree: Load traversed subdirs when creating parents

I'm working on enhancing the ostree-rs-ext test suite and I hit
a bug where walking a mtree and creating a parent would fail to
load lazy intermediate directories, e.g.:

/ -> usr -> bin

If we walked we'd load `/` but keep `usr` lazy, and then invalidation
would crash because it wasn't loaded.

If we're going to mutate a subdir, we need to have all the parents
loaded.

I know this is missing tests, but...it's a bit tedious to do with
the existing C tests.  Eventually soon we'll execute on merging
all 3 repos, and better share test suites.