Auto merge of #2040 - alexcrichton:dedupe, r=brson

I removed this in 68014ab8 thinking it wasn't necessary, which it technically
isn't for correctness but it ends up leading to absurdly long command lines to
the compiler for larger projects. This also changes the `Vec` to be a `BTreeSet`
to have sorting and deduplication as we go along which should be much faster
than waiting to sort until the very end.

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