df: fix output if input path is device name

[cakebaker]

Currently, when providing a device name as a path, you get some output like:

$ ./target/debug/coreutils df /dev/sda2 --out=source,target
Filesystem     Mounted on
dev            /dev

With this PR, the output is now the same as the output from GNU df:

$ ./target/debug/coreutils df /dev/sda2 --out=source,target
Filesystem     Mounted on
/dev/sda2      /boot

Fixes #3246

[anastygnome]

Good, but there's room for improvement :

find returns the first match (so index 0) or None, which we can then catch using the or_else clause.

This allows for further compile time optimisations ;)

[anastygnome]

Woops, didn't get the whole range.

[anastygnome]

Why not do something like this instead ?

    let maybe_mount_point: Option<&MountInfo> = mounts
        .iter()
        .find(|mi| mi.dev_name.eq(&path.to_string_lossy()));

    maybe_mount_point.or_else(|| {
        mounts
            .iter()
            .filter(|mi| path.starts_with(&mi.mount_dir))
            .max_by_key(|mi| mi.mount_dir.len())
    })
}

[cakebaker]

Thanks for your suggestion. This is indeed a much cleaner solution and I switched to it in the recent push.