History improvements (#337)

* Simplified redoOnce.

* Help mental model.

* Move clear redo stack where it belongs.

* Not needed anymore as we check for same state.

src/index.tsx

@@ -291,7 +291,7 @@ export class App extends React.Component<{}, AppState> {
     } else if (event[META_KEY] && event.code === "KeyZ") {
       if (event.shiftKey) {
         // Redo action
-        const data = history.redoOnce(elements);
+        const data = history.redoOnce();
         if (data !== null) {
           elements = data;
         }
@@ -1026,7 +1026,6 @@ export class App extends React.Component<{}, AppState> {
     this.saveDebounced();
     if (history.isRecording()) {
       history.pushEntry(history.generateCurrentEntry(elements));
-      history.clearRedoStack();
     }
   }
 }