showCache result for showSimilarRestricts the cloud-sourced metadata (from showCacheCloud()) to the same
function as the current call, then rbinds it onto the local shownCache,
de-duplicating on cacheId/tagKey/tagValue. Pure (no I/O) so the merge
behaviour can be tested without Google Drive.
mergeShownCacheCloud(shownCache, cloudShown, .functionName = NULL)A combined cache data.table.
The local showCache() data.table.
The cloud metadata data.table from showCacheCloud().
The function name to keep from cloudShown (or NULL for all).