feat: implement reference-cache for faster checkouts

- Add `reference-cache` input to action.yml
- Introduce `GitCacheHelper` for bare clone cache management
- Prevent race conditions with `proper-lockfile` and atomic directory renames
- Support iterative submodule caching and robust relative URL resolution
- Append to `info/alternates` preserving existing alternate references
- Add fallback to standard clone on submodule cache failure
- Add unit tests for `GitCacheHelper`

Signed-off-by: Michael Wyraz <mw@brick4u.de>

__test__/git-auth-helper.test.ts

@@ -1047,8 +1047,10 @@ async function setup(testName: string): Promise<void> {
     lfsFetch: jest.fn(),
     lfsInstall: jest.fn(),
     log1: jest.fn(),
+    referenceAdd: jest.fn(),
     remoteAdd: jest.fn(),
     removeEnvironmentVariable: jest.fn((name: string) => delete git.env[name]),
+    execGit: jest.fn(),
     revParse: jest.fn(),
     setEnvironmentVariable: jest.fn((name: string, value: string) => {
       git.env[name] = value
@@ -1173,7 +1175,8 @@ async function setup(testName: string): Promise<void> {
     sshUser: '',
     workflowOrganizationId: 123456,
     setSafeDirectory: true,
-    githubServerUrl: githubServerUrl
+    githubServerUrl: githubServerUrl,
+    referenceCache: ''
   }
 }