gem-diff.rb

#!/usr/bin/env ruby

require 'rubygems/command'
require 'uri'

class GitHub
  def initialize(gem_name, versions)
    @gem_name = gem_name
    old_version, new_version = *versions
    @old_version = Gem::Version.new(old_version)
    @new_version = Gem::Version.new(new_version)
  end

  def homepage
    Gem::Specification.find_by_name(@gem_name).homepage.to_s.sub(/\Ahttps?/, 'https')
  end

  def diff_url(suffix = '')
    "#{homepage}/compare/#{suffix}#{@old_version}...#{suffix}#{@new_version}"
  end

  def github?
    homepage.match(/\Ahttps?:\/\/github\.com\//)
  end
end

class Diff
  def initialize
    @gems = Hash.new { |h, k| h[k] = [] }
  end

  def push(line)
    parse1 line
  end
  alias_method :<<, :push

  def parse1(line)
    regexp = /\A
               ([+-])  # sign
               \s*
               (\S+) # gem_name
               \s*
               \(
                 ([\d.]+) # version_number
               \)
               \s*
             \z/x
    sign, gem_name, version_number = line.match(regexp).captures
    @gems[gem_name] << version_number
  rescue NoMethodError
  end

  def each(&block)
    @gems.each do |gem_name, versions|
      yield(gem_name, versions) if block_given?
    end
  end
end

@diff = Diff.new
ARGF.readlines.each do |line|
  @diff << line
end

@diff.each do |gem_name, versions|
  github = GitHub.new(gem_name, versions)
  next unless github.github?
  begin
    open(github.diff_url)
    `open #{github.diff_url}`
  rescue
    open(github.diff_url('v'))
    `open #{github.diff_url('v')}`
  end
end

Interesting! I just found this. I built something very similar this weekend. I thought you might be interested and maybe even like to contribute:

https://github.com/teeparham/gemdiff