Comment 1 for bug 1006870

Note also that this just allows us to stop early. sync_exchange itself should be sending the "this is the revision we think you are at" and the server should abort if the transaction id doesn't match. It can do better detection anyway (because it probably has the history of transaction ids, rather than just the tip).

We can mark this WontFix if we feel it isn't worth the extra logic at this level.