Updated git diff statement (home-assistant#4877)

Author: cdce8p

source/developers/development_testing.markdown

@@ -54,7 +54,7 @@ $ pydocstyle homeassistant/core.py
 $ py.test tests/test_core.py
 ```
 
-You can also run linting tests against all changed files, as reported by `git diff upstream/dev... --name-only`, using the `lint` script:
+You can also run linting tests against all changed files, as reported by `git diff upstream/dev... --diff-filter=d --name-only`, using the `lint` script:
 
 ```bash
 $ script/lint