def committers(commit_id = nil)
      committers = ActorCollection.new
      if commit_id.nil?
        each { |commit| committers << commit.committer }
      elsif key? commit_id
        committers << self[commit_id].committer
      end
      committers
    end
def committers(commit_id = nil)
      committers = ActorCollection.new
      if commit_id.nil?
        each { |commit| committers << commit.committer }
      elsif key? commit_id
        committers << self[commit_id].committer
      end
      committers
    end
Returns the committers of all or a specific commit in this collection

 @param [Object] commit_id The ID of the commit, if only the committer of
        a specific commit should be returned
 @return [ActorCollection] All committers of the commits in this
         collection or the committer of a specific commit
 @see Commit#committer