mv: add --exchange to atomically swap two paths

[sylvestre]

Should make test tests/mv/mv-exchange.sh pass

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/mv/mv-exchange is no longer failing!
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/cut/cut-huge-range is now passing!

[codspeed-hq]

Merging this PR will degrade performance by 90.94%

⚠️ 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

❌ 6 regressed benchmarks
✅ 325 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_ascii_c_locale	16.7 ms	1,723.3 ms	-99.03%
❌	Simulation	sort_ascii_utf8_locale	16.1 ms	1,274.4 ms	-98.73%
❌	Memory	sort_ascii_c_locale	4.6 MB	81.8 MB	-94.37%
❌	Memory	sort_ascii_utf8_locale	4.6 MB	52.4 MB	-91.21%
❌	Simulation	mv_force_overwrite	62.8 ms	66 ms	-4.84%
❌	Simulation	mv_single_file	62.6 ms	65.4 ms	-4.26%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing sylvestre:mv-exchange (8015178) with main (2c2f1b0)²}

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. ↩

2. No successful run was found on main (32952d8) during the generation of this report, so 2c2f1b0 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[cakebaker]

I would also test the scenario where source and target are the same file.