helper/schema: diffs for sets should include the full set [GH-457]

Prior to this, the diff only contained changed set elements. The issue
with this is that `getSet`, the internal function that reads a set from
the ResourceData, expects that each level (state, config, diff, etc.)
has the _full set_ information. This change was done to fix merging
issues.

Because of this, we need to make sure the full set is visible in the
diff.