cache-tree: do not generate empty trees as a result of all i-t-a subentries

If a subdirectory contains nothing but i-t-a entries, we generate an
empty tree object and add it to its parent tree. Which is wrong. Such
a subdirectory should not be added.

Note that this has a cascading effect. If subdir 'a/b/c' contains
nothing but i-t-a entries, we ignore it. But then if 'a/b' contains
only (the non-existing) 'a/b/c', then we should ignore 'a/b' while
building 'a' too. And it goes all the way up to top directory.

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -325,6 +325,7 @@ static int update_one(struct cache_tree *it,
 		const unsigned char *sha1;
 		unsigned mode;
 		int expected_missing = 0;
+		int contains_ita = 0;
 
 		path = ce->name;
 		pathlen = ce_namelen(ce);
@@ -341,7 +342,8 @@ static int update_one(struct cache_tree *it,
 			i += sub->count;
 			sha1 = sub->cache_tree->sha1;
 			mode = S_IFDIR;
-			if (sub->cache_tree->entry_count < 0) {
+			contains_ita = sub->cache_tree->entry_count < 0;
+			if (contains_ita) {
 				to_invalidate = 1;
 				expected_missing = 1;
 			}
@@ -380,6 +382,12 @@ static int update_one(struct cache_tree *it,
 			continue;
 		}
 
+		/*
+		 * "sub" can be an empty tree if all subentries are i-t-a.
+		 */
+		if (contains_ita && !hashcmp(sha1, EMPTY_TREE_SHA1_BIN))
+			continue;
+
 		strbuf_grow(&buffer, entlen + 100);
 		strbuf_addf(&buffer, "%o %.*s%c", mode, entlen, path + baselen, '\0');
 		strbuf_add(&buffer, sha1, 20);

t/t2203-add-intent.sh

@@ -99,5 +99,19 @@ test_expect_success 'cache-tree does not ignore dir that has i-t-a entries' '
 	)
 '
 
+test_expect_success 'cache-tree does skip dir that becomes empty' '
+	rm -fr ita-in-dir &&
+	git init ita-in-dir &&
+	(
+		cd ita-in-dir &&
+		mkdir -p 1/2/3 &&
+		echo 4 >1/2/3/4 &&
+		git add -N 1/2/3/4 &&
+		git write-tree >actual &&
+		echo $EMPTY_TREE >expected &&
+		test_cmp expected actual
+	)
+'
+
 test_done