Implement GIT_DIFF_INCLUDE_UNREADABLE_AS_UNTRACKED

src/diff.c

@@ -995,6 +995,9 @@ static int handle_unmatched_new_item(
 	}
 
 	else if (nitem->mode == GIT_FILEMODE_UNREADABLE) {
+		if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_INCLUDE_UNREADABLE_AS_UNTRACKED))
+			delta_type = GIT_DELTA_UNTRACKED;
+		else
 			delta_type = GIT_DELTA_UNREADABLE;
 	}