code: def get_commits(self, since_sha=None):
          
          assert self.tempdir
  
          cmd = ['git', 'log', '--first-parent', '--reverse', COMMIT_FORMAT]
          if since_sha:
              commits = [self.get_commit(since_sha)]
              cmd.append('{}..HEAD'.format(since_sha))
          else:
              commits = []
              cmd.append('HEAD')
  
          output = cmd_output(*cmd, cwd=self.tempdir)
  
          for sha, date in chunk_iter(output.splitlines(), 2):
              commits.append(Commit(sha, int(date)))
  
          return commits
docs: Returns a list of Commit objects.
  
          Args:
             since_sha - (optional) A sha to search from