Add configurable timeout and retry for git network operations

Add per-attempt timeout (default 300s) and Kubernetes probe-style retry
configuration for git fetch, lfs-fetch, and ls-remote. New action inputs:
timeout, retry-max-attempts, retry-min-backoff, retry-max-backoff.

Fixes https://github.com/actions/checkout/issues/631

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

__test__/git-directory-helper.test.ts

@@ -506,6 +506,8 @@ async function setup(testName: string): Promise<void> {
     tryReset: jest.fn(async () => {
       return true
     }),
-    version: jest.fn()
+    version: jest.fn(),
+    setTimeout: jest.fn(),
+    setRetryConfig: jest.fn()
   }
 }