On dirty tree, `cargo fix` should tell the user what changes would be applied

[epage]

Problem

The intention of requiring --allow-dirty is if the user might not like the edits, making it hard for the user to separate their changes from cargo fixs changes.

Right now, the user is out of luck for knowing if its safe, unless they want to commit.

With #6040, they could separately do a dry-run but that is still having to go through extra steps.

Proposed Solution

What if cargo fix, when detecting a dirty tree, acted as if --dry-run was passed in (without showing the "aborted due to dry-run" message) but still error out in the end with a message about the tree being dirty.

Notes

No response