rebase: explicitly track collapses as fold A collapse is a large fold. It is now explicitly tracked (at the cleanupnodes API level).

rebase: use tuple as `replacement` keys Now that `cleanupnodes` support tuples as key, we update the rebase code to use them. No changes in the replacement tracked are introduced yet.

rebase: expand a long "one-liner" When a one-liner gets 3 lines longs, it lose its expressivity benefits. We expand it into a simple for loop. This makes future changes of the code in that area clearer.

cleanupnodes: drop special casing around prune markers (API) The `cleanupnodes` has logic to skip the creation of "prune" markers if the changeset is already obsolete. This feels strange and gets in the way of code changes to tracks folds. Now that callers no longer request such prune, we can drop this logic. In many cases, pruning through cleanupnodes should be replaced by internal phase usage.

rebase: don't try to prune obsolete changeset already in the destination With similar motivations to the previous changesets, we stop marking changeset from pruning when it is not the command intention. In this case, we still need to distinguish between the strip and the obsolete case.

histedit: don't cleanup nodes already disposed of If something else took care of these temporary nodes, we don't need to do anything about it. This less liberal usage of pruning through cleanup nodes will help us further cleanup on the road to explicitly tracks folds.