Feature request: buildifier diff mode that would show changes applied by --lint=fix

It would be great to have some way be able to run `buildifier` with `--mode=diff` which takes into account the diffs that would be applied by `--lint=fix`.

This would be useful for having a CI check that makes sure `buildifier --lint=fix ...` is run before code can be merged. Currently, this can only be done by actually running `buildifier --lint=fix` and then running `git diff` to see if there were any changes, but this is not ideal because it touches the working tree.

Start at the buildifier command-line handling for --mode and --lint=fix, then trace how the formatter currently applies fixes. Compare the proposed --mode=diff behavior with the existing workflow of running buildifier and git diff. Done means CI can show whether --lint=fix would change files without touching the working tree.

Written by the indexing model from the issue text.