don't apply unchanged attributes from legacy diffs

If a legacy diff has equal old and new values, don't apply the diff.
These would show up in sets, because of the overall change in set key.

terraform/diff.go

@@ -457,6 +457,11 @@ func (d *InstanceDiff) ApplyToValue(base cty.Value, schema *configschema.Block)
 			continue
 		}
 
+		// sometimes helper/schema gives us values that aren't really a diff
+		if diff.Old == diff.New {
+			continue
+		}
+
 		attrs[attr] = diff.New
 	}