unexpand: increase buf size

[oech3]

Closes #11787

[codspeed-hq]

Merging this PR will degrade performance by 6.73%

❌ 2 regressed benchmarks
⏩ 354 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	unexpand_many_lines[100000]	135.7 ms	145.4 ms	-6.73%
❌	Simulation	unexpand_large_file[10]	284.2 ms	304.7 ms	-6.73%

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_{Comparing oech3:unexpand-buf (ee2a6d8) with main (b2619df)²}

Footnotes

1. 354 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. ↩

2. No successful run was found on main (717140a) during the generation of this report, so b2619df was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩