refactor: avoid unnecessary HashSet::contains

[xtqqczze]

Replace the contains + insert pair with a single HashSet::insert call.

This reduces two hash table lookups to one while preserving existing behavior.

[codspeed]

Merging this PR will degrade performance by 3.58%

❌ 1 regressed benchmark
✅ 24 untouched benchmarks
⏩ 422 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	du_all_wide_tree[(5000, 500)]	37.1 ms	38.5 ms	-3.58%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing xtqqczze:refactor-du-hashset-contains (cf8fd9a) with main (cacdd8d)}

Footnotes

1. 422 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[xtqqczze]

@samueltardieu I'm surprised Clippy doesn't suggest replacing a contains + insert pair with HashSet::insert.

[xtqqczze]

Benchmark variance tracked by #14984.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/id/setgid. tests/id/setgid is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks!