(core) hide long sequences of unchanged rows in diffs

Summary:
It can be hard to find changes, even when highlighted, in a table with many rows.  This diff replaces long sequences of unchanged rows with a row containing "..."s.

With daff, I found that it is important to do this for sequences of unchanged columns also, but not tackling that yet.

Test Plan: added test

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2666

app/client/models/ClientColumnGetters.ts

@@ -1,3 +1,4 @@
+import * as DataTableModel from 'app/client/models/DataTableModel';
 import {ColumnGetters} from 'app/common/ColumnGetters';
 import * as gristTypes from 'app/common/gristTypes';
 
@@ -9,12 +10,30 @@ import * as gristTypes from 'app/common/gristTypes';
  */
 export class ClientColumnGetters implements ColumnGetters {
 
-  constructor(private _tableModel: any) {
+  // If the "unversioned" option is set, then cells with multiple
+  // versions will be read as a single version - the first version
+  // available of parent, local, or remote.  This can make sense for
+  // sorting, so cells appear in a reasonably sensible place.
+  constructor(private _tableModel: DataTableModel, private _options: {
+    unversioned?: boolean} = {}) {
   }
 
   public getColGetter(colRef: number): ((rowId: number) => any) | null {
     const colId = this._tableModel.docModel.columns.getRowModel(Math.abs(colRef)).colId();
-    return this._tableModel.tableData.getRowPropFunc(colId);
+    const getter = this._tableModel.tableData.getRowPropFunc(colId);
+    if (!getter) { return getter || null; }
+    if (this._options.unversioned && this._tableModel.tableData.mayHaveVersions()) {
+      return (rowId) => {
+        const value = getter(rowId);
+        if (value && gristTypes.isVersions(value)) {
+          const versions = value[1];
+          return ('parent' in versions) ? versions.parent :
+            ('local' in versions) ? versions.local : versions.remote;
+        }
+        return value;
+      };
+    }
+    return getter;
   }
 
   public getManualSortGetter(): ((rowId: number) => any) | null {