Add individual parents to text dump

[benjeffery]

Fixes #1828

[jeromekelleher]

LGTM

[codecov]

Codecov Report

Merging #1884 (7a766a7) into main (c352421) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1884      +/-   ##
==========================================
- Coverage   93.13%   93.12%   -0.01%     
==========================================
  Files          27       27              
  Lines       24917    24918       +1     
  Branches     1095     1095              
==========================================
- Hits        23207    23206       -1     
- Misses       1675     1677       +2     
  Partials       35       35              

Flag	Coverage Δ
c-tests	92.21% <ø> (ø)
lwt-tests	89.14% <ø> (ø)
python-c-tests	68.58% <0.00%> (-0.01%)	⬇️
python-tests	98.72% <100.00%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
python/tskit/trees.py	97.78% <100.00%> (-0.13%)	⬇️

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