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

[bduffany]

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.