Add support for SHA256 repositories

New input parameter to specify the git object format when initializing a
git repository.

README.md

@@ -126,6 +126,9 @@ 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: ''
+
+    # Specify the Git object format for repository initialization (sha1, sha256).
+    object-format: ''
 ```
 <!-- end usage -->