ancestor_track_id not filled correctly

[sindhu-ku]

parent_track_id seems to match track_id but not ancestor_track_id:

>>> [p.track_id() for p in tree.particle_pcluster_branch.as_vector()]
[2689, 5207, 5210, 5211, 6730, 2905, 2900, 2907, 2903, 2913, 2927, 2924, 6158, 6736, 6735, 6731, 6739, 6741]
>>> [p.parent_track_id() for p in tree.particle_pcluster_branch.as_vector()]
[2689, 5207, 5210, 5211, 6730, 2900, 2689, 2905, 2900, 2903, 2913, 2913, 5210, 6735, 6731, 6730, 6736, 6739]
>>> [p.ancestor_track_id() for p in tree.particle_pcluster_branch.as_vector()]
[0, 1, 2, 3, 4, 0, 0, 0, 0, 0, 0, 0, 2, 4, 4, 4, 4, 4]