fastcheck: do not test context in pycodestyle
    
    `git diff` shows also context lines by default. When passed to pycodestyle
    it can produce errors unrelated to changed lines. It prevents running of
    subsequent checks.
    
    Limiting context to 0 lines by `git diff -U0` enables to test only the
    modified lines and allows to run subsequent checks.
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>