sort: use locale_cmp when all selectors have equal ordering

[lauchimoon]

Fixes #13319.

When all selectors return Ordering::Equal in compare_by, the last resort was to use byte-by-byte comparison, ignoring the locale.

The change now resorts to use locale_cmp if i18n-collator is enabled.

[codspeed-hq]

Merging this PR will degrade performance by 11.73%

⚠️ 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

❌ 4 regressed benchmarks
✅ 327 untouched benchmarks
⏩ 46 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	sort_numeric[500000]	1 s	1.4 s	-26.32%
❌	Simulation	sort_key_field[500000]	767 ms	836.9 ms	-8.36%
❌	Simulation	sort_case_insensitive[500000]	251.6 ms	268.9 ms	-6.42%
❌	Simulation	sort_dictionary_order[500000]	1 s	1.1 s	-3.92%

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 lauchimoon:numeric-fix (de6f5a2) with main (cd80f63)}

Footnotes

1. 46 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:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.