T296814 image suggestions delta bugfix

First we work out the commonswiki data for the current snapshot, and then we get the data from the previous snapshot and compute the delta ... but we don't specify the wiki, so it gets all the data from the previous snapshot for all commons AND non-commons wikis, and compares that with just new commonswiki data. And then does similar when working out the delta for non-commons wikis (gets ALL data from the previous snapshot)

... and as a result the deltas are a mess. This is a fix