(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/server/lib/HashUtil.ts

@@ -0,0 +1,46 @@
+import { DocState } from 'app/common/UserAPI';
+
+/**
+ *
+ * Helper class to support a small subset of git-style references for state hashes:
+ *   HEAD = the most recent state
+ *   [HASH]^1 = the parent of [HASH]
+ *   [HASH]~1 = the parent of [HASH]
+ *   [HASH]~2 = the grandparent of [HASH]
+ *   [HASH]^1^1 = the grandparent of [HASH]
+ *   [HASH]~3 = the great grandparent of [HASH]
+ * For git, where commits have multiple parents, "~" refers to the first parent,
+ * and "^1" also refers to the first parent.  For grist, there are only first parents
+ * (unless/until we start tracking history across merges).
+ *
+ */
+export class HashUtil {
+
+  /**
+   * To construct, provide a list of states, most recent first.
+   */
+  constructor(private _state: DocState[]) {}
+
+  /**
+   * Find the named hash in the list of states, allowing for aliases.
+   * Returns an index into the list of states provided in constructor.
+   */
+  public hashToOffset(hash: string): number {
+    const parts = hash.split(/([~^][0-9]*)/);
+    hash = parts.shift() || '';
+    let offset = hash === 'HEAD' ? 0 : this._state.findIndex(state => state.h === hash);
+    if (offset < 0) { throw new Error('Cannot read hash'); }
+    for (const part of parts) {
+      if (part === '^' || part === '^1') {
+        offset++;
+      } else if (part.startsWith('~')) {
+        offset += parseInt(part.slice(1) || '1', 10);
+      } else if (part === '') {
+        // pass
+      } else {
+        throw new Error('cannot parse hash');
+      }
+    }
+    return offset;
+  }
+}