text: from difflib import unified_diff
  
  from pyvcs.exceptions import FileDoesNotExist
  
  def generate_unified_diff(repository, changed_files, commit1, commit2):
      diffs = []
      for file_name in changed_files:
          try:
              file1 = repository.file_contents(file_name, commit1)
          except FileDoesNotExist:
              file1 = ''
          try:
              file2 = repository.file_contents(file_name, commit2)
          except FileDoesNotExist:
              file2 = ''
          diffs.append(unified_diff(
              file1.splitlines(), file2.splitlines(), fromfile=file_name,
              tofile=file_name, fromfiledate=commit1, tofiledate=commit2
          ))
      return '\n'.join('\n'.join(map(lambda s: s.rstrip('\n'), diff)) for diff in diffs)
  