This sanity check was added I6922b597 to make SubmitStrategyOp able to
fix changes that appeared merged in the repo but not the db. However,
that's not the only case where MergeSuperSet is used: when checking
the ETag of revisions for a merged change, we do want to use the empty
ChangeSet, since once a change is merged we no longer need to consider
dependent changes.
Move the sanity check to MergeOp where it belongs, avoiding an ISE in
GetRevisionActions. Add test coverage for this case.
Change-Id: I2a57553b30eab85355f161914f79b6f30fff4e45