Bug 957721 - Transform mach into a shell script that reexecutes itsel…

…f with python. r=mshal

mach

@@ -1,8 +1,14 @@
-#!/usr/bin/env python
+#!/bin/sh
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+# The beginning of this script is both valid shell and valid python,
+# such that the script starts with the shell and is reexecuted with
+# the right python.
+'''which' python2.7 > /dev/null && exec python2.7 "$0" "$@" || exec python "$0" "$@"
+'''
+
 from __future__ import print_function, unicode_literals
 
 import os