perf(split): reuse buffer in chunked splitting loop

[CrazyRoka]

This PR optimizes the memory allocation strategy in the split utility when handling chunked output.

Moved the Vec initialization outside of the for loop to avoid unnecessary malloc and free calls, leading to a more efficient execution path

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/follow-name (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 13.25%

⚡ 1 improved benchmark
✅ 283 untouched benchmarks
⏩ 38 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	split_number_chunks	286.3 µs	252.8 µs	+13.25%

---

_{Comparing CrazyRoka:reuse-split-buffer (bc5bebc) with main (26a29ca)}

Footnotes

1. 38 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. ↩

[sylvestre]

Merging this PR will improve performance by 13.26%

bravo!

[sylvestre]

@samueltardieu maybe it is hard but maybe something that clippy could detect ? :)