title: flake8 --diff not working from directory except repository root
body: <p>Repository tree is shown as above</p>
  
  <pre><code>.
  ├── __init__.py
  └── source
      ├── __init__.py
      └── main.py
  </code></pre>
  
  <p><strong>main.py</strong> file</p>
  
  <pre><code>def add(a, b):
      return a + b
  </code></pre>
  
  <p>When I change file <code>main.py</code> to</p>
  
  <pre><code>def add(a, b):
      return a +b
  </code></pre>
  
  <p>and make <code>git diff -U0 | flake8 --diff</code> from repository root it shows me</p>
  
  <blockquote>
    <p>source/main.py:2:15: E225 missing whitespace around operator</p>
  </blockquote>
  
  <p>but when I make the same command from <code>source</code> folder it doesn't show anything</p>
  
  <p>At the same moment <code>git diff -U0</code> shows an identical result inside root repository and <code>source</code> folder</p>
  
  <pre><code>diff --git a/source/main.py b/source/main.py
  index 4693ad3..fd47298 100644
  --- a/source/main.py
  +++ b/source/main.py
  @@ -2 +2 @@ def add(a, b):
  -    return a + b
  +    return a +b
  </code></pre>