In git-review, use /usr/bin/python instead of /usr/local/bin/python.
This will be better for most systems. This only matters when running
git-review out of the source tree, as setup.py updates this path during
installation. I haven't used "/usr/bin/env python" for now, since then
I can't pass in the -tt argument. However, we could still switch to
that later if there is a compelling reason to.
In setup.py, invoke python as "/usr/bin/env python", to find python from
the user's $PATH. setup.py will modify the installed git-review to
invoke python from the same path found by setup.py.