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>

src/git-source-settings.ts

@@ -59,6 +59,11 @@ export interface IGitSourceSettings {
    */
   showProgress: boolean
 
+  /**
+   * The path to a local directory used as a reference cache for Git clones
+   */
+  referenceCache: string
+
   /**
    * Indicates whether to fetch LFS objects
    */