SimpleCov config to add groups for tracking changed files in a commit and a branch. Originally from an email by @fredwu.

simplecov.rb

if ENV['COV']
  require 'simplecov'
  SimpleCov.start 'rails' do
    add_group 'This Commit' do |source_file|
      `git ls-files --exclude-standard --others \
        && git diff --name-only \
        && git diff --name-only --cached`.split("\n").detect do |filename|
        source_file.filename.ends_with?(filename)
      end
    end

    base_commit = `git merge-base master HEAD`.chomp
    add_group 'This Branch' do |source_file|
      `git diff #{base_commit}..HEAD --name-only`.split("\n").detect do |filename|
        source_file.filename.ends_with?(filename)
      end
    end
  end
end