Simplify population semantics in split_edges

[jeromekelleher]

Closes #2334

[codecov]

Codecov Report

Merging #2348 (6aaa04b) into main (a1b1e5d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2348      +/-   ##
==========================================
- Coverage   93.27%   93.27%   -0.01%     
==========================================
  Files          28       28              
  Lines       26768    26755      -13     
  Branches     1217     1216       -1     
==========================================
- Hits        24969    24956      -13     
  Misses       1766     1766              
  Partials       33       33              

Flag	Coverage Δ
c-tests	92.25% <ø> (-0.01%)	⬇️
lwt-tests	89.05% <ø> (ø)
python-c-tests	71.47% <66.66%> (+<0.01%)	⬆️
python-tests	98.84% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
c/tskit/trees.c	95.03% <ø> (-0.01%)	⬇️
python/_tskitmodule.c	90.67% <100.00%> (-0.01%)	⬇️
python/tskit/trees.py	98.27% <100.00%> (-0.01%)	⬇️

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[benjeffery]

LGTM!