Fix: Use old cycles during unmerge

[haneslinger]

In contrast to #3904, here's the new workflow

Okay, so we've got a property present in two cycles, one has been merged into:

see?

Now if we unmerge the merged one:

We've one got one new property:

[haneslinger]

@kflemin @axelstudios the failing test here imply this intented behavoir. why? Is it still?

[kflemin]

I don't have any historical context for this one. @nllong? @axelstudios? Basically, what we are trying to do here is: when we unmerge 2 properties, we want to associate one of the unmerged properties back to the "old" existing property and create a new property for the other unmerged property. This is in contrast to what happens now: when a property is "unmerged" into 2, it creates 2 new properties.