investigate pre-push hook.

[technoweenie]

Git 1.8.2 has a pre_push hook. We should use that to run git media push. Let's use that so we don't have to worry about wrapping git push.

• Add pre-push hook Add the pre-push hook setup #62
• git media init adds hook if run inside a repository. Add the pre-push hook setup #62
• The git media clean/smudge filters also install hooks when run inside a repository. Auto install hooks #65

The goal is for this to work:

# the smudge filter downloads media files, and installs the hook locally.
$ git clone https://github.com/github/big-repo

# the clean filter queues the file to be uploaded, and installs the hook locally.
# this is useful in case the `clone` didn't have any big media files.
$ git add some-big-file.mov

$ git commit -m "whatevs"
$ git push # runs pre-push