warn about pushing "/xy" to a path and overwriting it.

[matthiaskrgr]

let mut x = PathBuf::from("/foo");
x.push("/bar");
println!("x: {}", x.display()); // "/bar"

I just ran into a situation where I did something like this, and it resulted in the path being
/bar instead of wanted /foo/bar.
What I actually wanted was

x.push("bar");

so that the path becomes /foo/bar

Could we warn about pushing an element to a pathbuf that has the file system root in it, or something like this

[flip1995]

This Lint can be implemented in 3 steps:

1. Match on MethodCall push
2. Match first (self) argument of push to be PathBuf
3. Check the &str that is pushed, whether it contains the file system root