Contextual diff lines checked

pycodestyle --diff is great! More programs should accept diffs as input.

Unfortunately it appears to consider lines in the diff that are merely context. Consider the following diff:

diff --git a/example.py b/example.py
index 955628d..b6c2b66 100644
--- a/example.py
+++ b/example.py
@@ -1,2 +1,5 @@
 def invalid_but_irrelevant():
   print("I am invalid but also irrelevant")
+
+def invalid_but_relevant():
+  print("I am invalid and relevant")

(Also available as a gist.)

Line 2 (print("I am invalid but also irrelevant")) was not added in this diff, but pycodestyle still finds the problem with its indentation:

user@host:~/gists/98f18739ab30fb69d6154364dc1eee7a$ git diff HEAD~1 | pycodestyle --diff
./example.py:2:3: E111 indentation is not a multiple of four
./example.py:4:1: E302 expected 2 blank lines, found 1
./example.py:5:3: E111 indentation is not a multiple of four

I expect --diff to only consider lines added by a diff; otherwise code that's problematic but outside the scope of a change causes that change to fail linting checks.

Reproduce the example by piping git diff HEAD~1 to pycodestyle --diff, then trace the --diff input handling in the repository's single Python file. Done means context lines such as example.py:2 are ignored while added lines still produce the expected errors.