dirname: get rid of needless Cow

[ChrisDenton]

All the returns are Borrowed so let's just return a &[u8] directly.

[oech3]

any chance for clippy detection?

[ChrisDenton]

There's an issue but not implementation yet: rust-lang/rust-clippy#11328

[ChrisDenton]

Oh, wait, that's only for String::new. I can't see anything more generally for Cow::borrowed.

[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/date/resolution (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/pipe-f (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/symlink (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will improve performance by 31.27%

⚡ 1 improved benchmark
✅ 241 untouched benchmarks
⏩ 113 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	cp_recursive_deep_tree[(120, 4)]	699.2 KB	532.7 KB	+31.27%

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_{Comparing ChrisDenton:cow (bdcba3b) with main (22bd036)}

Footnotes

1. 113 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. ↩

[cakebaker]

Thanks for your PR!