hg-cpython: fallback when encountering an unknown matcher At this point in the process, nothing user-visible has happened, it is still safe to fallback. This can happen now that we're going to be using "container matchers" like unionmatcher and intersectionmatcher. This is easier and less error-prone than recursive checking beforehand since only the presence of a transformation case will allow the process to continue.

auto-upgrade: add an option to silence the safe-mismatch message For well tested case, the message can get in the way, so we add a way to disable it.