Figure out how to force `cargo fix` to run

[alexcrichton]

Moved over from rust-lang/rustfix#79, I hear @killercup has a fix in mind!

[alexcrichton]

Bumping this to the RC1 milestone

[nrc]

ping @killercup are you still planning to work on this?

[alexcrichton]

I sort of forget the original rationale for this, but one articulation of a use case is:

• First, run cargo fix --edition
• Realize that you have a ton of warnings, and it turns out that cargo fix has a bug in it. A big message was printed at the top indicating so, and you'd like to see it again.
• Rerun cargo fix --edition and this time redirect the output to a file so you can read over it later

The second fix doesn't actually do anything!

[dwijnand]

Looks like #5944 might be doing the trick.