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>

README.md

@@ -160,6 +160,12 @@ Please refer to the [release page](https://github.com/actions/checkout/releases/
     # running from unless specified. Example URLs are https://github.com or
     # https://my-ghes-server.example.com
     github-server-url: ''
+
+    # Path to a local directory used as a reference cache for Git clones. Over time,
+    # this directory will contain bare clones of the checked-out repositories (and
+    # their submodules). Using this significantly reduces network bandwidth and speeds
+    # up clones.
+    reference-cache: ''
 ```
 <!-- end usage -->