(core) Prevent stray 'compare' param from appearing in some snapshot links

Summary:
Some slight difference appeared, possibly during rebase, about including
compare parameters in URLs.

Test Plan: Browser test was failing, now passes

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2683

app/client/ui/DocHistory.ts

@@ -59,7 +59,9 @@ export class DocHistory extends Disposable implements IDomComponent {
     // We include urlState().state to preserve the currently selected page.
     function setLink(snapshot: DocSnapshot, compareDocId?: string) {
       return dom.attr('href', (use) => urlState().makeUrl({
-        ...use(urlState().state), doc: snapshot.docId, params: {compare: compareDocId}}));
+        ...use(urlState().state), doc: snapshot.docId,
+        params: (compareDocId ? {compare: compareDocId} : {})
+      }));
     }
 
     const snapshots = Observable.create<DocSnapshot[]>(owner, []);