Made auto-review base rev ID handling smarter.

* This adds an altBaseRevId hidden param and uses it to deal
  with autoreview for rollback/undo of the latest revision(s)
  to an unreviewed one, where the latest was reviewed. We can
  trust that the user did not make the page worse in this case.

Bug: 43966
Change-Id: Ibd36b60df6cff0d04e035b81855b2789b22a9240
Merge "Made auto-review base rev ID handling smarter."