(core) add more detail to /compare endpoint

Summary:
 * Extends `/api/docs/docId1/compare/docId2` endpoint with a `detail=1` option to include details of what changed in the document content.
 * Adds an `/api/docs/docId/compare?left=HASH&right=HASH` endpoint for comparing two versions of a single document. This is needed to implement the extension to `/api/docs/docId1/compare/docId2`.
 * Adds a `HashUtil` class to allow hash aliases like `HEAD` and `HEAD~`.

Everything is a bit crude:
 * Changes are expressed as ActionSummary objects, which aren't fully fleshed out.
 * Extra data about formula columns is inserted in an inflexible way.

This is extracted and cleaned up from https://phab.getgrist.com/D2600.

Test Plan: added tests

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2614

app/common/ActionSummary.ts

@@ -1,5 +1,6 @@
 import {CellDelta, TabularDiff, TabularDiffs} from 'app/common/TabularDiff';
 import toPairs = require('lodash/toPairs');
+import { RowRecord } from 'app/plugin/GristData';
 
 /**
  * An ActionSummary represents the overall effect of changes that took place
@@ -90,6 +91,13 @@ export interface TableDelta {
   /** Partial record of cell-level changes - large bulk changes not included. */
   columnDeltas: {[colId: string]: ColumnDelta};
   columnRenames: LabelDelta[];  /** a list of column renames/additions/removals */
+
+  /*
+   * A snapshot of row content for added and updated rows, including formula columns.
+   * Not included in regular summaries, but used in comparisons.  Hopefully this
+   * can evaporate in future.
+   */
+  finalRowContent?: {[rowId: number]: RowRecord};
 }
 
 /**