Rollup merge of rust-lang#74137 - ehuss:update-cargo, r=ehuss

Update cargo

6 commits in fede83ccf973457de319ba6fa0e36ead454d2e20..4f74d9b2a771c58b7ef4906b2668afd075bc8081
2020-07-02 21:51:34 +0000 to 2020-07-08 17:13:00 +0000
- Disable long_file_names test if not supported on Windows. (rust-lang/cargo#8469)
- Add support for deserializing enums in config files (rust-lang/cargo#8454)
- Write GNU tar files, supporting long names. (rust-lang/cargo#8453)
- Don't overwrite existing `rustdoc` args with --document-private-items (rust-lang/cargo#8449)
- Add some help about rustup's +toolchain syntax. (rust-lang/cargo#8455)
- Update metadata man page. (rust-lang/cargo#8451)