Merge branch 'rr/t4041-cleanup'

* rr/t4041-cleanup:
  t4041 (diff-submodule-option): modernize style
  t4041 (diff-submodule-option): rewrite add_file() routine
  t4041 (diff-submodule-option): parse digests sensibly
  t4041 (diff-submodule-option): don't hardcode SHA-1 in expected outputs