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/input-helper.ts

@@ -161,5 +161,9 @@ export async function getInputs(): Promise<IGitSourceSettings> {
   result.githubServerUrl = core.getInput('github-server-url')
   core.debug(`GitHub Host URL = ${result.githubServerUrl}`)
 
+  // Reference Cache
+  result.referenceCache = core.getInput('reference-cache')
+  core.debug(`Reference Cache = ${result.referenceCache}`)
+
   return result
 }