All objects were being parented to a single instance of each parent
object, instead of their respective instances, when using dupliverts or
dupligroups.
Behavior was caused by the `persistent_id[0]` (vertex/face id) being
ignored when computing `parent_gh` hash, which caused all instances to
have the same hash, and thus only the first one was included.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D2370
99c5c8befc