Parsing of `cargo run --features a b` has regressed

[alexcrichton]

Given a package that has the feature a but not the feature b, I get:

$ cargo run --features a b
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/wut b`
Hello, world!
$ cargo +beta run --features a b
error: Package `wut v0.1.0 (/home/alex/code/wut)` does not have these features: `b`

$ cargo --version
cargo 1.37.0 (9edd08916 2019-08-02)
$ cargo +beta --version
cargo 1.38.0-beta (23ef9a4ef 2019-08-20)

I suspect this is an accidental regression from #7084