Allow branch labels from second tree

Branch labels were already rendered correctly on the RHS tree if the key
was present in the LHS tree, so all that's required is to add label keys
which are only observed in the RHS tree to appear in the sidebar branch
label selector.

Closes #1780

src/components/controls/choose-branch-labelling.js

@@ -11,7 +11,10 @@ import Toggle from "./toggle";
 @connect((state) => ({
   selected: state.controls.selectedBranchLabel,
   showAll: state.controls.showAllBranchLabels,
-  available: state.tree.availableBranchLabels,
+  available: Array.from(
+    (new Set(state.tree.availableBranchLabels))
+      .union(new Set(state.treeToo?.availableBranchLabels ?? []))
+  ),
   canRenderBranchLabels: state.controls.canRenderBranchLabels
 }))
 class ChooseBranchLabelling extends React.Component {