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

The 'tox -epep8' command takes a long time as it checks
every single file in the Nova git repository (~1,400 at
time of writing).

This makes tox use a simple wrapper around flake8 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 1,400 source files.

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

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

Change-Id: I157d1ccb883ca02402eee51fd7d6a50f86079389

tools/flake8wrap.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+#
+# 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

@@ -27,7 +27,7 @@ downloadcache = ~/cache/pip
 
 [testenv:pep8]
 commands =
-  flake8 {posargs}
+  bash tools/flake8wrap.sh {posargs}
 
 [testenv:py34]
 setenv = {[testenv]setenv}