tox: make it possible to run pep8 on current patch only

This makes tox use a simple wrapper around flake8 like
Nova, which can be told to restrict the check to only
files changed in the current command. This can be invoked
in a simple manner with 'tox -epep8 -- -HEAD'. Since most
commits only touch a handful of files, this will usually
be far faster than checking all source files.

To check an entire branch for bisectability it can be
automated via

  git rebase -i master -x 'tox -epep8 -- -HEAD'

Change-Id: I75b1cfe3198b1217f8b25877714a4af47ae7069f

tools/flake8wrap.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+#
+# A simple wrapper around flake8 which makes it possible
+# to ask it to only verify files changed in the current
+# git HEAD patch.
+#
+# Intended to be invoked via tox:
+#
+#   tox -epep8 -- -HEAD
+#
+
+if test "x$1" = "x-HEAD" ; then
+    shift
+    files=$(git diff --name-only HEAD~1 | tr '\n' ' ')
+    echo "Running flake8 on ${files}"
+    diff -u --from-file /dev/null ${files} | flake8 --diff "$@"
+else
+    echo "Running flake8 on all files"
+    exec flake8 "$@"
+fi

tox.ini

@@ -25,7 +25,7 @@ commands = {toxinidir}/tools/states_to_dot.py -f {toxinidir}/doc/source/images/s
 [testenv:pep8]
 whitelist_externals = bash
 commands =
-  flake8 {posargs}
+  bash tools/flake8wrap.sh {posargs}
   # Run bashate during pep8 runs to ensure violations are caught by
   # the check and gate queues.
   {toxinidir}/tools/run_bashate.sh {toxinidir}/devstack