dirname handles paths ending in `/.` differently from GNU coreutils

[jfmc]

This bug completely breaks https://github.com/ciao-lang/ciao installation on Ubuntu 25.10.

The Rust implementation of dirname is giving different output than GNU coreutils for paths that end with /.

What I'm seeing

On GNU coreutils:

$ dirname "a/b/."
a/b

On rust coreutils (Ubuntu 25.10):

$ dirname "a/b/."
a

It works on Ubuntu 25.05 and macOS but breaks on Ubuntu 25.10 because of this dirname behavior difference.

According to POSIX, dirname should "remove the suffix from the last '/' to the end of string". For a/b/., the last / is between b and ., so removing everything after that should give you a/b.

[cakebaker]

This issue has already been fixed on main with #8936 :

$ cargo run -q dirname "a/b/."
a/b

Anyway, thanks for reporting.