Teach diff that modified submodule directory is dirty

A diff run in superproject only compares the name of the commit object
bound at the submodule paths.  When we compare with a work tree and the
checked out submodule directory is dirty (e.g. has either staged or
unstaged changes, or has new files the user forgot to add to the index),
show the work tree side as "dirty".

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2029,9 +2029,14 @@ static int populate_from_stdin(struct diff_filespec *s)
 static int diff_populate_gitlink(struct diff_filespec *s, int size_only)
 {
 	int len;
-	char *data = xmalloc(100);
+	char *data = xmalloc(100), *dirty = "";
+
+	/* Are we looking at the work tree? */
+	if (!s->sha1_valid && is_submodule_modified(s->path))
+		dirty = "-dirty";
+
 	len = snprintf(data, 100,
-		"Subproject commit %s\n", sha1_to_hex(s->sha1));
+		       "Subproject commit %s%s\n", sha1_to_hex(s->sha1), dirty);
 	s->data = data;
 	s->size = len;
 	s->should_free = 1;

t/t4027-diff-submodule.sh

@@ -32,7 +32,8 @@ test_expect_success setup '
 		cd sub &&
 		git rev-list HEAD
 	) &&
-	echo ":160000 160000 $3 $_z40 M	sub" >expect
+	echo ":160000 160000 $3 $_z40 M	sub" >expect &&
+	subtip=$3 subprev=$2
 '
 
 test_expect_success 'git diff --raw HEAD' '
@@ -50,6 +51,87 @@ test_expect_success 'git diff-files --raw' '
 	test_cmp expect actual.files
 '
 
+expect_from_to () {
+	printf "%sSubproject commit %s\n+Subproject commit %s\n" \
+		"-" "$1" "$2"
+}
+
+test_expect_success 'git diff HEAD' '
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subtip $subprev &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (work tree)' '
+	echo >>sub/world &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subtip $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (index)' '
+	(
+		cd sub &&
+		git reset --hard &&
+		echo >>world &&
+		git add world
+	) &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subtip $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (untracked)' '
+	(
+		cd sub &&
+		git reset --hard &&
+		git clean -qfdx &&
+		>cruft
+	) &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subtip $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (work tree, refs match)' '
+	git commit -m "x" sub &&
+	echo >>sub/world &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subprev $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (index, refs match)' '
+	(
+		cd sub &&
+		git reset --hard &&
+		echo >>world &&
+		git add world
+	) &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subprev $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
+test_expect_success 'git diff HEAD with dirty submodule (untracked, refs match)' '
+	(
+		cd sub &&
+		git reset --hard &&
+		git clean -qfdx &&
+		>cruft
+	) &&
+	git diff HEAD >actual &&
+	sed -e "1,/^@@/d" actual >actual.body &&
+	expect_from_to >expect.body $subprev $subprev-dirty &&
+	test_cmp expect.body actual.body
+'
+
 test_expect_success 'git diff (empty submodule dir)' '
 	: >empty &&
 	rm -rf sub/* sub/.git &&