According to gitformat-index.txt [1], "mode" is 32 bits, but we've only documented 16 bits. I tried poking around read-cache.c, and my impression is that other 16 are just NUL. If so, it would be worth documenting that they're unused, especially since we documented unused bits right in that section. [1] https://github.com/git/git/blob/master/Documentation/gitformat-index.txt#L84
