How do I unstage a staged deletion?

i-do-not-know.rb

# Assume my-file.txt has been commited previously
repo = Rugged::Repository.discover(some_dir)
File.delete("my-file.txt")

# This will stage the file deletion
repo.index.remove("my-file.txt") # This doesn't work
repo.index.write

# How do I "unstage" this deletion?
repo.index.add("my-file.txt") # This doesn't work... what will?