Re: [WIP v1 2/4] mv: add check_dir_in_index() and solve general dir check issue

> I read and think about this part a few times, but I'm still confused.
>
> As Victoria pointed out earlier, and I quote, "Sparse directories *only* appear
> when you're using a sparse index, so, theoretically, this shouldn't ever
> come up (and thus isn't testable) until you're using a sparse index."
> So I'm not so sure what do you mean by putting "full index" and "sparse
> directory" together.
>
> Thus, I go ahead and try to detect a directory that is outside of
> sparse-checkout cone, without sparse-index enabled.
>
> I found a problem that if you use cache_name_pos() to do this
> detection, I imagined the following example (I'm trying to imitate an
> output of "git ls-files -t"):
>
> H a
> H b
> S d/file1
> H e/file1
>
> So in this index, I use cache_name_pos() to find a directory "c/". I imagine the
> the value returned would be -3, which indicates this directory would be inserted
> at index position 2. However, the cache entry at position 2 is
> "d/file1", which is
> marked with SKIP_WORKTREE, and this fact cannot guarantee that "c/" is
> a sparse directory, since ''c/" is not in the index per se.
>
> Probably I'm missing something, or I'm just dumb.