Add allow staged to cargo fix

[JoshBrudnak]

Added the --allow-staged command line flag to cargo fix. #5737

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Thanks! It's a bit difficult to see what's going on here, could the rustfmt changes be left in a separate commit?

[bors]

☔ The latest upstream changes (presumably #5943) made this pull request unmergeable. Please resolve the merge conflicts.