Not exactly finished. 
It is useful to not try and choose a revision which the current directory doesn't exist at. 
Use svn log #{repo} to obtain the earliest revision. Or just choose the HEAD

#!/usr/bin/env ruby

repository = ARGV.first
abort("Please provide a repository") if repository.nil?

puts "Using repository: #{repository.inspect}"

print "Enter the revision to start history from: "
start = $stdin.gets
unless start == "HEAD"
  start = Integer(start)
end

puts "Revision to start at: #{start}"
system('svn', 'co', '-r', start.to_s, repository, "trunk") || abort("Could not svn checkout")

Dir.chdir("trunk") do
    system("hg", "init")
    File.open(".hgignore", "w") do |f|
        f.puts '.svn'
    end
    system('hg', 'add', '-X', '.svn', '-X' '**/.svn')
    system('hg', 'ci', '-m' 'First import')
    system('hg', 'tag', '-l', "svn.#{start}")
    system('hgpullsvn')
end