Doesn't find git root when inside a submodule

No hooks are installed when the repo has been cloned as a submodule.  In this case, the `.git` file is plain text pointing to a directory under the parent `.git/modules/...`.

I see code to handle this case, but it is unreachable because the condition here triggers first: https://github.com/observing/pre-commit/blob/master/install.js#L26