commit_date: code cleanup and readme updates

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

README.md

@@ -45,6 +45,7 @@ ___
     * [`{{base_ref}}`](#base_ref)
     * [`{{is_default_branch}}`](#is_default_branch)
     * [`{{date '<format>' tz='<timezone>'}}`](#date-format-tztimezone)
+    * [`{{commit_date '<format>' tz='<timezone>'}}`](#commit_date-format-tztimezone)
   * [Major version zero](#major-version-zero)
   * [JSON output object](#json-output-object)
   * [Overwrite labels and annotations](#overwrite-labels-and-annotations)
@@ -891,13 +892,14 @@ Default `tz` is UTC.
 
 #### `{{commit_date '<format>' tz='<timezone>'}}`
 
-Returns the date when current git commit is committed.
-rendered by its [moment format](https://momentjs.com/docs/#/displaying/format/).
+Returns the date when the current git commit is committed, rendered by its
+[moment format](https://momentjs.com/docs/#/displaying/format/). It falls back
+to the current date if the commit date is not available.
 
 Default `tz` is UTC.
 
-| Expression                                   | Output example                          |
-|----------------------------------------------|-----------------------------------------|
+| Expression                                          | Output example                          |
+|-----------------------------------------------------|-----------------------------------------|
 | `{{commit_date 'YYYYMMDD'}}`                        | `20200110`                              |
 | `{{commit_date 'dddd, MMMM Do YYYY, h:mm:ss a'}}`   | `Friday, January 10th 2020, 3:25:50 pm` |
 | `{{commit_date 'YYYYMMDD-HHmmss' tz='Asia/Tokyo'}}` | `20200110-093000`                       |

__tests__/context.test.ts

@@ -5,9 +5,9 @@ import * as path from 'path';
 import {Context} from '@actions/github/lib/context';
 import {Git} from '@docker/actions-toolkit/lib/git';
 import {GitHub} from '@docker/actions-toolkit/lib/github';
+import {Toolkit} from '@docker/actions-toolkit/lib/toolkit';
 
 import {ContextSource, getContext, getInputs, Inputs} from '../src/context';
-import {Toolkit} from '@docker/actions-toolkit/lib/toolkit';
 
 const toolkit = new Toolkit({githubToken: 'fake-github-token'});
 

__tests__/meta.test.ts

@@ -2,10 +2,11 @@ import {beforeEach, describe, expect, jest, test} from '@jest/globals';
 import * as fs from 'fs';
 import * as path from 'path';
 import * as dotenv from 'dotenv';
+
+import {Context} from '@actions/github/lib/context';
 import {GitHub} from '@docker/actions-toolkit/lib/github';
 import {Toolkit} from '@docker/actions-toolkit/lib/toolkit';
 import {GitHubRepo} from '@docker/actions-toolkit/lib/types/github';
-import {Context} from '@actions/github/lib/context';
 
 import {ContextSource, getContext, getInputs, Inputs} from '../src/context';
 import {Meta, Version} from '../src/meta';