(core) visualize simple differences between documents

Summary:
Render simple differences between documents.

 * Show cell changes.
 * Show cell conflicts.
 * Show row additions/deletions.

Doesn't support any schema changes, and is untested in the presence of schema changes.  Any widgets that access row data without using `cells` fields won't receive correct data.

Not addressed:
 * Rendering conflicts in mixed row addition/updating/deleting.
 * Column additions/deletions, option changes, etc.
 * Document level changes.
 * Table and column renames (though anticipated in ActionSummary structure).
 * Page-level changes.
 * Drawing attention to changes (marking changed pages+views, suppressing
   unchanged rows, etc).
 * Rendering differences in views other than GridView.
 * Adding UI for initiating a comparison.
 * Editing while comparing.

Replaces {D2600}

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2618

app/common/DocActions.ts

@@ -6,6 +6,12 @@
 import { CellValue } from 'app/plugin/GristData';
 export { CellValue, RowRecord } from 'app/plugin/GristData';
 
+// Part of a special CellValue used for comparisons, embedding several versions of a CellValue.
+export type CellVersions =
+  { parent: CellValue, remote: CellValue } |
+  { parent: CellValue, local: CellValue }  |
+  { parent: CellValue, local: CellValue, remote: CellValue };
+
 import map = require('lodash/map');
 
 export type AddRecord = ['AddRecord', string, number, ColValues];

app/common/gristTypes.ts

@@ -1,4 +1,4 @@
-import {CellValue} from 'app/common/DocActions';
+import { CellValue, CellVersions } from 'app/common/DocActions';
 import isString = require('lodash/isString');
 
 // tslint:disable:object-literal-key-quotes
@@ -22,6 +22,7 @@ export const enum GristObjCode {
   Exception       = 'E',
   Pending         = 'P',
   Unmarshallable  = 'U',
+  Versions        = 'V',
 }
 
 export const MANUALSORT = 'manualSort';
@@ -110,6 +111,14 @@ export function isRaisedException(value: CellValue): boolean {
   return getObjCode(value) === GristObjCode.Exception;
 }
 
+/**
+ * Returns whether a value (as received in a DocAction) represents a group of versions for
+ * a comparison or conflict.
+ */
+export function isVersions(value: CellValue): value is [GristObjCode.Versions, CellVersions] {
+  return getObjCode(value) === GristObjCode.Versions;
+}
+
 /**
  * Returns whether a value (as received in a DocAction) represents a list or is null,
  * which is a valid value for list types in grist.
@@ -130,7 +139,8 @@ function isNumberOrNull(v: CellValue) { return isNumber(v) || v === null; }
 function isBoolean(v: CellValue) { return typeof v === 'boolean' || v === 1 || v === 0; }
 
 // These values are not regular cell values, even in a column of type Any.
-const abnormalValueTypes: string[] = [GristObjCode.Exception, GristObjCode.Pending, GristObjCode.Unmarshallable];
+const abnormalValueTypes: string[] = [GristObjCode.Exception, GristObjCode.Pending,
+                                      GristObjCode.Unmarshallable, GristObjCode.Versions];
 
 function isNormalValue(value: CellValue) {
   return !abnormalValueTypes.includes(getObjCode(value)!);