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.

diff --git a/diff-lib.c b/diff-lib.c
index 346cac6..0b8c58d 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -105,9 +105,6 @@ int run_diff_files(struct rev_info *revs, unsigned int 
                int changed;
                unsigned dirty_submodule = 0;
-               if (diff_can_quit_early(&revs->diffopt))
-                       break;
                if (!ce_path_match(ce, &revs->prune_data))
IWAMOTO Toshihiro
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