diff: make sure work tree side is shown as 0{40} when different

Ping Yin noticed that "git diff-index --raw" shows 0{40} when work tree
has submodule difference, but "git diff --raw" didn't correctly do so.

There was a mistake in the diffcore_skip_stat_unmatch() that was meant to
clean up the stat-only difference for running diff between the index and
work tree and diff between the tree and the work tree, to cause it re-read
from the submodule repository HEAD.  When ce_stat_match() says work tree
is different, we should always say 0{40} on the work tree side.

This patch fixes the issue, and adds tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -3187,11 +3187,8 @@ static void diffcore_apply_filter(const char *filter)
 static int diff_filespec_is_identical(struct diff_filespec *one,
 				      struct diff_filespec *two)
 {
-	if (S_ISGITLINK(one->mode)) {
-		diff_fill_sha1_info(one);
-		diff_fill_sha1_info(two);
-		return !hashcmp(one->sha1, two->sha1);
-	}
+	if (S_ISGITLINK(one->mode))
+		return 0;
 	if (diff_populate_filespec(one, 0))
 		return 0;
 	if (diff_populate_filespec(two, 0))

t/t4027-diff-submodule.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+
+test_description='difference in submodules'
+
+. ./test-lib.sh
+. ../diff-lib.sh
+
+_z40=0000000000000000000000000000000000000000
+test_expect_success setup '
+	test_tick &&
+	test_create_repo sub &&
+	(
+		cd sub &&
+		echo hello >world &&
+		git add world &&
+		git commit -m submodule
+	) &&
+
+	test_tick &&
+	echo frotz >nitfol &&
+	git add nitfol sub &&
+	git commit -m superproject &&
+
+	(
+		cd sub &&
+		echo goodbye >world &&
+		git add world &&
+		git commit -m "submodule #2"
+	) &&
+
+	set x $(
+		cd sub &&
+		git rev-list HEAD
+	) &&
+	echo ":160000 160000 $3 $_z40 M	sub" >expect
+'
+
+test_expect_success 'git diff --raw HEAD' '
+	git diff --raw --abbrev=40 HEAD >actual &&
+	diff -u expect actual
+'
+
+test_expect_success 'git diff-index --raw HEAD' '
+	git diff-index --raw HEAD >actual.index &&
+	diff -u expect actual.index
+'
+
+test_expect_success 'git diff-files --raw' '
+	git diff-files --raw >actual.files &&
+	diff -u expect actual.files
+'
+
+test_done