Path::relative_from() doesn't account for case insensitivity on Windows

[nukep]

This has resulted in an issue in Cargo where rustdocs cannot be generated on Windows machines:
rust-lang/cargo#1393

use std::path::Path;
fn main() {
    let p = Path::new(r"C:\src\rust\cargo-test\test\Cargo.toml");
    println!("{:?}", p.relative_from(r"C:\src\rust\cargo-test\test"));
    println!("{:?}", p.relative_from(r"c:\src\rust\cargo-test\test"));
}

Some("Cargo.toml")
None

[alexcrichton]

cc @aturon