Work around incorrect $GIT_DIR on Heroku

It appears that `$GIT_DIR="."` in the deploy environment, while we're not running in `/app/tmp/repo.git`, so any call to `git` fails.

This workaround hard-codes the absolute path, and frankly it seems like a bad idea but I could not come up with anything better, and our deploys are failing so this is kind of urgent.

@kr if you happen to know what happened to `GIT_DIR`, that could improve on this maybe.