comm: fix false sorting warnings when reading from FIFOs/process substitution

[sylvestre]

No description provided.

[codspeed-hq]

Merging this PR will degrade performance by 5.09%

⚡ 1 improved benchmark
❌ 1 regressed benchmark
✅ 280 untouched benchmarks
⏩ 38 skipped benchmarks¹

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	cp_large_file[16]	114.1 KB	120.2 KB	-5.09%
⚡	Memory	numfmt_large_numbers_si[10000]	4.8 MB	4.6 MB	+4.08%

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_{Comparing sylvestre:comm (06d22f4) with main (038a08b)}

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. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/follow-name. tests/tail/follow-name is passing on 'main'. Maybe you have to rebase?

[sylvestre]

dupe of #9545