backend/retry: Overwrite failed uploads instead of deleting them
backend/retry: Overwrite failed uploads instead of deleting them

For backends which are able to atomically replace files, we just can
overwrite the old copy, if it is necessary to retry an upload. This has
the benefit of issuing one operation less and might be beneficial if a
backend storage, due to bugs or similar, could mix up the order of the
upload and delete calls.

restic/restic,restic/restic