When an attempt to revise an entity results in no new revisions, it should fail.

[acoffman]

When proposing a revision set, we track which revisions are newly created and which revisions previously existed. In the case when all revisions already existed, no activity record should be generated and the request should fail.

Furthermore, in cases when some revisions are new and others pre-existing, only the new revisions should be linked to the activity record.