bug with git-diff --quiet

I found "git-diff --quiet" returns a zero exit status even if there's a change. The following sequence reproduces the bug:

  $ mkdir foo
  $ cd foo
  $ git init
  $ echo a > a.txt
  $ echo b >b.txt
  $ git add ?.txt
  $ git commit
  $ echo b >> b.txt
  $ touch a.txt
  $ git diff --quiet; echo $?
  
Interestingly, if you issue "git-diff --quiet" again, it returns the
expected exit status 1.

The problem is in the optimization code in run_diff_files(). The function finds a.txt has different stat(2) data from .git/index and calls diff_change(), which sets DIFF_OPT_HAS_CHANGES. As the flag makes diff_can_quit_early() return 1, run_diff_files()'s loop finishes without calling diff_change() for b.txt.

Then, diffcore_std() examines diff_queued_diff and clears DIFF_OPT_HAS_CHANGES, because a.txt is unchanged. This is how a change in b.txt is ignored by git-diff --quiet.

Here's a obvious fix for this bug, but I think you can find a better fix. Thanks in advance.