Rollup merge of rust-lang#84765 - ehuss:update-cargo, r=ehuss

Update cargo

7 commits in 4369396ce7d270972955d876eaa4954bea56bcd9..f3e13226d6d17a2bc5f325303494b43a45f53b7f
2021-04-27 14:35:53 +0000 to 2021-04-30 21:50:27 +0000
- Fix problem with metrics test. (rust-lang/cargo#9440)
- Show transfer rate when fetching/updating registry index (rust-lang/cargo#9395)
- Fix collision doc tests randomly failing. (rust-lang/cargo#9434)
- Add missing tracking issues and unstable docs. (rust-lang/cargo#9429)
- Fix dep-info files emitting paths relative to deps' roots (rust-lang/cargo#9421)
- Upgrade to GitHub-native Dependabot (rust-lang/cargo#9428)
- Only deny the `unused_mut` lint (rust-lang/cargo#9425)

Author: JohnTitor