--match_dir does not work properly

[glennmatthews]

The --match_dir is not respected properly when multiple directories exist:

> mkdir bar
> mkdir bat
> mkdir baz
> touch bar/bar.py
> touch bat/bat.py
> touch baz/baz.py
> pep257 --match-dir=bar
./bar/bar.py:1 at module level:
        D100: Docstring missing
./baz/baz.py:1 at module level:
        D100: Docstring missing

bar is correctly matched, and bat is correctly excluded, but baz is incorrectly included.

I have a candidate fix for this - will submit pull request.