Factories should ideally use first available Person.name

[Kit Randel]

Rather than throwing a NameAlreadyTaken exception, it would be convenient if factories (e.g. factory.makeBranchMergeProposal) could either ensure that the Person.name namespace is larger (introduce more randomness to the name to avoid collisions), or check that the name does not already exist (probably not possible without a big performance hit to tests).