fix generating a list of files/directories ignored by git

```git ls-files --others --ignore --exclude-standard --directory``` doesn't include the files that for example get ignored by this .gitignore line:

```
default/*
```

Without the ```--directory``` line it includes these files, but doesn't include:

```
folder/
```

So combine those 2 commands. The next line (```grep -xFvf```) requires an exact
match (```-x```). As a result the files in the 1st example were included in the
.etckeeper file.

Reference: http://stackoverflow.com/questions/466764/show-ignored-files-in-git#answer-1446609