Ref T12996. See also T9749. This option is the opposite of --lintall, and means "don't show warnings on changed lines".
However, it's confusing (and I think no one uses it), and we can reasonably infer what you mean if you give us --rev. Simplify this logic a bit and choose the "all / only changed" behavior based on the only sensible thing we can reasonably do given the flags.
As with all the other cruft here, I'm pretty sure this came from FB.