(core) Restoring context menu when comparing documents

Summary:
Fixing bug when comparing documents. User wasn't able
to open cell/column context menu due to javascript error.

Test Plan: Updated tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3397

app/client/models/DataTableModelWithDiff.ts

@@ -261,6 +261,10 @@ export class TableDataWithDiff {
     return this.core.getValue(rowId, colId);
   }
   public get tableId() { return this.core.tableId; }
+
+  public numRecords() {
+    return this.core.numRecords();
+  }
 }
 
 /**