findDecisionForProposition

The decision where propositionId shows up on either side of a merge: as the survivor, or as one of the retired members. Useful for a caller that just wants "what happened to this proposition" without caring which side it was on. A caller that specifically wants a retirement, undoing a single collapse say, should use findDecisionRetiring: this method can answer a decision where the id survived, which is the wrong one to undo against.