Re-add setuptools-git to setup.py.

In 81cbbcc4e01c81084c352f7fc1ecea149089a2b9 we removed:

  setup_requires=['setuptools-git>=0.4'],

from setup.py. This broke tarball generation in that some files are
now missing.

Fixes LP Bug #1118511.

Change-Id: I27a09e0d49befa3751031582d1b152677822041f