See the blog post http://blog.mattwynne.net/2013/07/24/how-much-should-i-refactor/

refactoring_commits.rb

Commit = Struct.new(:repo, :sha, :comment, :stats) do
  def refactoring?(previous_commit)
    return :spec_files_unchanged   if no_changes_to_specs
    return :comment                if refactoring_comment
    return false if result.nil? || result.failed > 0 || result.total == 0
    return :spec_results_unchanged if result == previous_commit.result
    return false
  end

  def no_changes_to_specs
    !stats.any? { |s| s.path =~ /^spec/ }
  end

  def refactoring_comment
    comment =~ /refactor/i
  end

  def result
    @result ||= repo.test(sha)
  end
end

Stat = Struct.new(:pct, :path)
Result = Struct.new(:total, :failed)

Repo = Struct.new(:path) do
  def dirstat
    `cd #{path} && git co master && git log --pretty=oneline --dirstat`.split("\n")
  end

  def test(sha)
    output = `cd #{path} && git checkout #{sha} && (rake || rspec)`
    results_match = output.match(/(\d+) examples, (\d+) failures/)
    if results_match
      total, failed = results_match.captures
      Result.new(total.to_i, failed.to_i)
    end
  end
end

class Log
  def initialize(repo_path)
    @repo = Repo.new(repo_path)
  end

  def commits
    commits = []
    @repo.dirstat.each do |line|
      match_commit = line.match(/^(\w+) (.*)$/)
      match_stat = line.match(/([\d\.]+)\% (.*)/)
      if match_commit
        sha, comment = match_commit.captures
        commit = Commit.new(@repo, sha, comment, [])
        commits << commit
      elsif match_stat
        pct, path = match_stat.captures
        stat = Stat.new(pct, path)
        commits.last.stats << stat
      else
        raise "Unable to parse this line:\n#{line}"
      end
    end
    commits
  end

  def refactoring_commits
    result = []
    commits.each_index do |index|
      previous = commits[index - 1]
      commit = commits[index]
      reason = commit.refactoring?(previous)
      result << [commit, reason] if reason
    end
    result
  end

end