Fix producing status when there is an unreadable folder in the working directory.

I don't really know my way around `libgit2` much, but I though I'd have a go at fixing the case of producing/enumerating status when there is an unreadable (644) folder in the working directory.

**TODO**
- [x] Write a failing test for `git_status_foreach_ext`
- [x] Make the tests pass :trolleybus: 
- [x] Write a test (and option) for not listing UNREADABLE files.
- [x] Add `GIT_DIFF_INCLUDE_UNREADABLE_AS_UNTRACKED`