input: Write a python function to tally votes for each commit.
  assert tally([Vote(commit='abc123', user='user1'), Vote(commit='abc123', user='user2'), Vote(commit='def456', user='user1')]) == {'abc123': {'user1', 'user2'}, 'def456': {'user1'}}
  assert tally([Vote(commit='xyz789', user='user3'), Vote(commit='xyz789', user='user4'), Vote(commit='lmn101', user='user2')]) == {'xyz789': {'user3', 'user4'}, 'lmn101': {'user2'}}
  assert tally([]) == {}
output: from collections import defaultdict
  
  class Vote:
      def __init__(self, commit, user):
          self.commit = commit
          self.user = user
  
  def tally(votes):
      result = defaultdict(set)
      for vote in votes:
          result[vote.commit].add(vote.user)
      return dict(result)