untracked cache: print stats with $GIT_TRACE_UNTRACKED_STATS

This could be used to verify correct behavior in tests

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

dir.c

@@ -1923,6 +1923,18 @@ int read_directory(struct dir_struct *dir, const char *path, int len, const stru
 	free_simplify(simplify);
 	qsort(dir->entries, dir->nr, sizeof(struct dir_entry *), cmp_name);
 	qsort(dir->ignored, dir->ignored_nr, sizeof(struct dir_entry *), cmp_name);
+	if (dir->untracked) {
+		static struct trace_key trace_untracked_stats = TRACE_KEY_INIT(UNTRACKED_STATS);
+		trace_printf_key(&trace_untracked_stats,
+				 "node creation: %u\n"
+				 "gitignore invalidation: %u\n"
+				 "directory invalidation: %u\n"
+				 "opendir: %u\n",
+				 dir->untracked->dir_created,
+				 dir->untracked->gitignore_invalidated,
+				 dir->untracked->dir_invalidated,
+				 dir->untracked->dir_opened);
+	}
 	return dir->nr;
 }