rewind: guard against rewinding to different predecessors at the same time
One case that is a bit weird, but should also be somehow handled, is when user asks to rewind to many predecessor sets, e.g. with 'allpredecessors(X)' or even with 'obsolete()'. Lets make rewind complain and abort by default. A new experimental config option is introduced to allow users to force such rewinds in case we have missed some valid use cases. It's also been agreed to not use --exact or --as-divergence flags to ignore this predecessors check.