message: hooks: Don't err if the changeset is empty.
diff: @@ -27,6 +27,8 @@ def main():
  subprocess.run(['tar', '-C', '{root}/prev'.format(**params), '-x'],
  stdin=git.pipe(['archive', HEAD, '--format=tar']))
  for line in git.git(['diff', '--cached', '--name-status', '--diff-filter=ACMR']).split('\n'):
  + if line == '':
  + continue
  status, file = line.split('\t', maxsplit=1)
  if status == 'A':
  before = None
  