Fix a case where we put an extra node in the SHAMapDiff

There's no need to add to a SHAMapDelta in any particular order since it's an ordered map. Plus, the code to do the insertion got the test wrong and would sometimes insert a node that shouldn't have been inserted. As we currently use the code only to create disputed transactions, this bug is effectively harmless but it would be very bad if this code were used for other purposes such as compact fetch packs.