Rename variables in tree_diff

[kayjan]

Description

Rename variables for understandability

Testing

Additional notes

Checklist

I have read through the contributing guidelines and ensured that

• I have added a descriptive title for this pull request.
• I have followed the convention and standards, and my code is checked for style and correctness.
• I have added test cases, and unit tests pass with 100% code coverage.
• I have updated the documentation and code docstrings.

Checklist (for reviewer)

• I have added label (breaking / enhancement / bug / documentation) to this pull request, if applicable.
• I will ensure this change is captured in the CHANGELOG.md file.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
bigtree	100%	100%	0	✔
bigtree.binarytree	100%	100%	0	✔
bigtree.dag	100%	100%	0	✔
bigtree.node	100%	100%	0	✔
bigtree.tree	100%	100%	0	✔
bigtree.utils	100%	100%	0	✔
Summary	100% (2389 / 2389)	100% (1002 / 1002)	0	✔

[github-actions]

Benchmark

Benchmark suite	Current: 5bfbb1f	Previous: 923488f	Ratio
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_10	15457.646820631942 iter/sec (stddev: 0.00001251805276944507)	16138.129477849408 iter/sec (stddev: 0.000004748575589810872)	1.04
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_100	661.9868827956101 iter/sec (stddev: 0.000055478537393098845)	657.7115511939296 iter/sec (stddev: 0.0000031609794429593763)	0.99
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_1000	11.753952315832922 iter/sec (stddev: 0.0033468756953929676)	11.701491184970395 iter/sec (stddev: 0.0032956993370807136)	1.00
tests/node/test_node_benchmark.py::test_node_benchmark_width_2_depth_10	67.33048918125192 iter/sec (stddev: 0.003333719549463631)	65.91483220333626 iter/sec (stddev: 0.0033048678250253627)	0.98
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_10_no_assertions	22904.2864223814 iter/sec (stddev: 0.000008746981594436825)	10838.504654946131 iter/sec (stddev: 0.00006853837537438091)	0.47
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_100_no_assertions	1745.1576021303895 iter/sec (stddev: 0.000003321068417397865)	1641.9602279447008 iter/sec (stddev: 0.0000013396137985196218)	0.94
tests/node/test_node_benchmark.py::test_node_benchmark_width_1_depth_1000_no_assertions	106.14177718058785 iter/sec (stddev: 0.004281918913179118)	114.45337736521977 iter/sec (stddev: 0.0034372627510679712)	1.08
tests/node/test_node_benchmark.py::test_node_benchmark_width_2_depth_10_no_assertions	87.18826587534852 iter/sec (stddev: 0.004132387325001829)	93.9704508601688 iter/sec (stddev: 0.0029928751304717674)	1.08

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