pep8 --diff also checks unmodified context

`pep8 --diff` doesn't only check modified code but also context which is a bad thing e.g. when using it in a git pre-commit hook.

Testcase to reproduce it:

**crap.py**
```
import sys

def foo( bar ):
    print "hello world"
    sys.exit (0)

def bar():
    pass


foo()
```

**crap2.py**
```
import sys

def foo( bar ):
    print "hello world"
    sys.exit (0)

def bar():
    print 'this function is not crap!'


foo()
```

**The resulting diff:**
```
--- crap.py     2013-06-25 14:34:36.677941391 +0200
+++ crap2.py    2013-06-25 14:34:39.366941926 +0200
@@ -5,7 +5,7 @@
     sys.exit (0)

 def bar():
-    pass
+    print 'this function is not crap!'


 foo()
```

**Output of `diff -u crap.py crap2.py | pep8 --diff`:**
```
./crap2.py:5:13: E211 whitespace before '('
./crap2.py:7:1: E302 expected 2 blank lines, found 1
```