Ref T13544. These flags change the behavior of the "arc lint" subprocess.
I believe there is no reason to ever use "arc diff --lintall". If you want to find extra warnings to fix, you can use "arc lint --lintall" to express this intent.
Use of "arc diff --only-new" almost certainly means your linters are raising messages at "error" severity which should instead be raised at "warning" severity. If you only care about fixing a particular type of error in changed code, it should be raised as a "warning". The correct remedy is to adjust the severity, not use "--only-new", which is a very broad, slow, complicated hammer.