Ignored folders with negated subfolders cause the parent to be noticed

**Steps:**

Create a repository containing this file/folder structure:

```
foo/
    bar/
        inner.txt
    outer.txt
```

Add the following to the repository's `.gitignore` (verbatim):

```
foo
!foo/bar
```

**Expected:** Everything under `foo` will be ignored (per core Git).
**Actual:** Neither file is ignored. This appears to be a regression.
