comm: compare lines with the locale collation

[luantaraschi]

sort orders lines with the locale collation and comm reads that order back
with a byte comparison. In a UTF-8 locale where the two disagree, comm rejects
a file sort has just produced and puts lines in the wrong column.

With LC_ALL=en_US.UTF-8, f1 holding a1 then a-b, which is the order
sort gives, and f2 holding a-b:

command	GNU	before	after
comm -12 f1 f2	a-b	nothing, the shared line is dropped	a-b
comm -13 f1 f2	nothing	a-b, a line that is in both files	nothing
comm -23 f1 f2	a1	a1 and a-b	a1
exit code	0	1	0

sort, join, ls and expr already go through uucore::i18n::collator, and
join, which has the same order check, reads this input correctly today. This
gives comm the same comparison. In the C locale the lines are still compared
as bytes, so nothing changes there.

#12280 also touches OrderChecker, but for a different thing: it changes when
the warning is printed, not how the order is measured.

Fixes #12912

#12972 was closed as a duplicate of that one. It is an AOSP build failing after
45 minutes with comm: file 1 is not in sorted order.

[Copilot]

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[github-actions]

Binary size comparison:

Individual binary size comparison VS main (threshold: >=5% AND >=4 KB).

Total size of compared binaries: 152.79 MB (+1.20 MB, +0.79%)

Significant per-binary changes:
  comm     1.13 MB ->    2.33 MB  (+1.20 MB, +106.57%)

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 4.14%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 356 untouched benchmarks
⏩ 50 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cksum_crc32b	40.3 ms	38.7 ms	+4.14%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing luantaraschi:fix/comm-locale-collation (cec5ab2) with main (0ecfd3b)}

Footnotes

1. 50 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. ↩