Auto merge of #58661 - ehuss:update-cargo, r=alexcrichton

Update cargo

5 commits in b33ce7fc9092962b0657b4c25354984b5e5c47e4..5c6aa46e6f28661270979696e7b4c2f0dff8628f
2019-02-19 18:42:50 +0000 to 2019-02-22 19:32:35 +0000
- convert unused doc comments to regular comments (rust-lang/cargo#6692)
- Add more about system library on whether to keep Cargo.lock (rust-lang/cargo#6685)
- Warn when excluding non-existing packages (rust-lang/cargo#6679)
- Incremental profile cleanup. (rust-lang/cargo#6688)
- Various cosmetic improvements (rust-lang/cargo#6687)