Reduce cliqueBranches role to sorting split commits by branchname

This is the missing commit in last MR

Since the code no longer cares about the order in which the branch cliques were encountered, stop recording that order. This enables populating cliqueBranches at the very last moment, just to sort it by name. In particular, no care is needed anymore to avoid the clique assigned to the base commit since it will not be in cliqueBranches.