Add support for SHA256 repositories

New input parameter 'repo-sha256' to set the Git object format
to "sha256" when initializing a Git repository.

README.md

@@ -126,6 +126,10 @@ 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: ''
+
+    # Set Git object format to "sha256" when initializing a Git repository.
+    # Default: false
+    repo-sha256: ''
 ```
 <!-- end usage -->