update.rb

#!/usr/bin/ruby

def try(cmd)
  puts cmd
  success = system(cmd)
  raise unless success
end

def current_commit()
  `git rev-parse head`.chomp
end

old_commit = ARGV[0]
raise "usage: `./update.rb <commit you want to update>" unless old_commit

tip = current_commit()

try("git add --all && git checkout #{old_commit} && git commit --amend --no-edit")
new_commit = current_commit()
try("git checkout - && git rebase --onto #{new_commit} #{old_commit}")