Merge branch 'js/diff-ni' (early part)

* 'js/diff-ni' (early part):
  diff --no-index: also imitate the exit status of diff(1)
  Fix typo: do not show name1 when name2 fails
  Teach git-diff-files the new option `--no-index`
  run_diff_{files,index}(): update calling convention.
  update-index: do not die too early in a read-only repository.
  git-status: do not be totally useless in a read-only repository.