`--allow-staged` flag

[zackmdavis]

As a generalization of --allow-dirty, there could be an --allow-staged flag that lets us run only if there are no unstaged changes in Git (and similarly if Mercurial, Pijul, or Fossil have an analogue of Git's staging area—I'm not familiar), the rationale being that content that has been git added isn't actually in danger of being permanently eaten by a poor suggestion application, because you can git checkout -- . to back out the changes.

[alexcrichton]

Closing in favor of rust-lang/cargo#5737 now that cargo fix has changed homes