Fix: Avoid infinite recursion when source and destinations are same while using cp -R

[Narasimha1997]

Proposing a solution for #3016
The infinite recursion can be prevented when running `cp -r <source_dir> <destination_dir> by handling these two cases:

1. Source and Destination refer to same canonicalized path, after resolving all symbolic links etc

# Example: 
cp -r /home/ubuntu/parent   /home/ubuntu/parent 

2. Canonicalized destination has a canonicalized source as prefix:

This can also lead to recursion, imagine there is a directory called child under /home/ubuntu/parent (i.e /home/ubuntu/parent/child, then the following example leads to recursion:

cp -r /home/ubuntu/parent /home/ubuntu/parent/child

The official GNU coreutil's cp handles both of these cases.

The above two cases can be solved by simply checking for case 2, i.e don't allow copy if destination has source in it's prefix.
I added a function:

pub fn path_has_prefix(p1: &Path, p2: &Path) -> io::Result<bool> {
    let pathbuf1 = canonicalize(p1, MissingHandling::Normal, ResolveMode::Logical)?;
    let pathbuf2 = canonicalize(p2, MissingHandling::Normal, ResolveMode::Logical)?;

    Ok(pathbuf1.starts_with(pathbuf2))
}

Which checks that and calling it in copy_directory function.

[sylvestre]

Fun, could you please add a test to make sure we don't regress?

[Narasimha1997]

@sylvestre added tests as you suggested.

[Narasimha1997]

Sorry clicked on Close with comment by mistake haha

[Narasimha1997]

@tertsdiepraam some merge issues with this branch

[tertsdiepraam]

Can you try to fix them? I think it's just that a test was added at the end of the file in both branches and that we should keep both.