Ensure that we can get a commit number from git

Currently, git describe fails with "fatal: No names found, cannot describe anything."

Author: invisibleroads

setup.py

@@ -201,12 +201,12 @@ def build_extensions(self):
  try:
  import subprocess
  try:
- pipe = subprocess.Popen(["git", "describe", "HEAD"],
+ pipe = subprocess.Popen(["git", "describe", "--always"],
  stdout=subprocess.PIPE).stdout
  except OSError:
  # msysgit compatibility
  pipe = subprocess.Popen(
- ["git.cmd", "describe", "HEAD"],
+ ["git.cmd", "describe", "--always"],
  stdout=subprocess.PIPE).stdout
  rev = pipe.read().strip()
  # makes distutils blow up on Python 2.7