dirname: use Cow::Borrowed to avoid unnecessary heap allocations

[sylvestre]

• remove useless comments

[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?

[ChrisDryden]

Did a search across the code base for other instances of this pattern and couldn't find any others, you'd think that slicing input data then calling .to_vec() in a Cow would be a Clippy rule

[sylvestre]

I did not but I can nerd snip @samueltardieu

[ChrisDryden]

I tried to look in clippy for this Cow stuff and for embedded cfg blocks #10296, couldn't even find suggestions in the issues queue

[samueltardieu]

Did a search across the code base for other instances of this pattern and couldn't find any others, you'd think that slicing input data then calling .to_vec() in a Cow would be a Clippy rule

Well, that may be required if you need the Cow to live longer than the slice.