Create setHasDiff helper and use it

The usage here is identical to how it'll work in Spaces

src/utils/sets.ts

@@ -0,0 +1,34 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Determines if two sets are different through a shallow comparison.
+ * @param a The first set. Must be defined.
+ * @param b The second set. Must be defined.
+ * @returns True if they are different, false otherwise.
+ */
+export function setHasDiff<T>(a: Set<T>, b: Set<T>): boolean {
+    if (a.size === b.size) {
+        // When the lengths are equal, check to see if either set is missing an element from the other.
+        if (Array.from(b).some(i => !a.has(i))) return true;
+        if (Array.from(a).some(i => !b.has(i))) return true;
+
+        // if all the keys are common, say so
+        return false;
+    } else {
+        return true; // different lengths means they are naturally diverged
+    }
+}