diff: do not quit early on stat-dirty files

When QUICK is set (i.e. with --quiet) we try to do as little work as
possible, stopping after seeing the first change. stat-dirty is
considered a "change" but it may turn out not, if no actual content is
changed. The actual content test is performed too late in the process
and the shortcut may be taken prematurely, leading to incorrect return
code.

Assume we do "git diff --quiet". If we have a stat-dirty file "a" and
a really dirty file "b". We break the loop in run_diff_files() and
stop after "a" because we have got a "change". Later in
diffcore_skip_stat_unmatch() we find out "a" is actually not
changed. But there's nothing else in the diff queue, we incorrectly
declare "no change", ignoring the fact that "b" is changed.

This also happens to "git diff --quiet HEAD" when it hits
diff_can_quit_early() in oneway_diff().

This patch does the content test earlier in order to keep going if "a"
is unchanged. The test result is cached so that when
diffcore_skip_stat_unmatch() is done in the end, we spend no cycles on
re-testing "a".

Reported-by: IWAMOTO Toshihiro <iwamoto@valinux.co.jp>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/diffcore.h b/diffcore.h
index 1c16c8595b..6b538bc2fb 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -70,6 +70,8 @@ struct diff_filepair {
 	unsigned broken_pair : 1;
 	unsigned renamed_pair : 1;
 	unsigned is_unmerged : 1;
+	unsigned done_skip_stat_unmatch : 1;
+	unsigned skip_stat_unmatch_result : 1;
 };
 #define DIFF_PAIR_UNMERGED(p) ((p)->is_unmerged)