ensure that tests can't shell out to our real `git` and `open` executables

Fake `git` and `open` executables are pushed to PATH in testing and error
out when invoked. This is because there are tests that shell out to `git init`
and `git clone`, or open the browser, and bugs and failures in these tests
would previously cause them to poke around the system uncontrolled.

test/fakebin/git

+#!/bin/sh
+if [[ $1 == "--version" ]]; then
+  echo "git version 1.7.0.4"
+else
+  echo "ERROR: git was called, but wasn't supposed to:" git $*
+  exit 1
+fi

test/fakebin/open

+#!/bin/sh
+echo "ERROR: open was called, but wasn't supposed to:" open $*
+exit 1
\ No newline at end of file

test/helper.rb

@@ -12,6 +12,10 @@ require 'hub/standalone'
 # We're looking for `open` in the tests.
 ENV['BROWSER'] = 'open'
 
+# Setup path with fake executables in case a test hits them
+fakebin_dir = File.expand_path('../fakebin', __FILE__)
+ENV['PATH'] = "#{fakebin_dir}:#{ENV['PATH']}"
+
 class Test::Unit::TestCase
   # Shortcut for creating a `Hub` instance. Pass it what you would
   # normally pass `hub` on the command line, e.g.