Blender Git Loki

Fix (unreported) asserts in `make_object_duplilist_real()`.

Code would try to add multiple time the same key in `parent_gh` (for this
ghash a lot of dupliobjects may generate same key).

Was making the tool unusable in debug builds.

Also optimise things a bit by avoiding creating parent_gh when only
`use_base_parent` is set.