Auto merge of #4390 - alexcrichton:read-hard-links, r=matklad

Use `same-file` to avoid unnecessary hard links

This is targeted at removing the need for a workaround in rust-lang/rust#39518,
allowing the main rust build system to move back to hard links which should be
much more efficient.

diff --cc Cargo.lock
Simple merge

diff --cc Cargo.toml
Simple merge

diff --cc src/cargo/lib.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/mod.rs
Simple merge