Xcode doesn't directly support a repo stored on S3 (it's something unique to JGit).
That said, you should be able to access a repo there by using a combination of FUSE for OS X and s3fs. You will need to install FUSE (which allows the use of filesystems from userland) and then s3fs (to mount the S3 bucket) and then mount the bucket containing the repo somewhere on your local filesystem. Once that's done, you'll be able to reference it using the local filesystem mountpoint.