pre-commit hook for `./x.py fmt` appears to not always know which files were changed.

[zachs18]

I tried this PR: #125283 (original push was just the first two commits, which have invalid formatting)

I expected to see this happen: git push pre-commit-hook should have noticed that the formatting for library/alloc/src/sync.rs was invalid in the pre-commit hook.

Instead, this happened: The push completed successfully.

I haven't been able to reproduce the issue (intentionally trying to push code with invalid formatting is caught by the pre-commit-hook in the times I've tried since).