rewind: add a --dry-run flag There are 3 cases handled separately in formatstatus(): - common-case rewind when we have X current successors rewinding to Y predecessors - rewinding with --as-divergence, when we don't obsolete current successors - rewinding pruned commits, when we don't have current successors to show In the common case, we might have sub-cases that change the output with --dry-run (but not the way the rewind operates): - more predecessors than successors (e.g. after a fold), in which case we simply use successorsmap, which maps successors to a set of predecessors - equal number of predecessors and successors, see the previous case - more successors than predecessors (e.g. after a split), in which case we use a reverse of successorsmap (rsm), which maps predecessors to a set of successors and exists only for the sake of --dry-run functionality These two dicts allow us to group rewind targets and output separate lines for separate predecessors<->successors relations.