[WIP] Validate hooks existence

Git, Make, Rake, Rails and several other tools share a behavior regarding their config files and folders. They look for them in parents folders until `/` is found.

An example to make things clearer:

```bash
$ pwd
/vagrant
$ ls .git
branches        config       FETCH_HEAD  hooks  info  objects    packed-refs      refs
COMMIT_EDITMSG  description  HEAD        index  logs  ORIG_HEAD  PULLREQ_EDITMSG
$ git status
# On branch master
nothing to commit (working directory clean)
$ cd config/
$ pwd
/vagrant/config
$ ls .git
ls: cannot access .git: No such file or directory
$ git status
# On branch master
nothing to commit (working directory clean)
```

Here you can see that even when `$PWD` in `/vagrant/config`, Git is still able to find `.git` which is the parent folder, `/vagrant`.

---

I'm concerned that the `Dir.pwd` in this line breaks this behavior. For example, if I use `ruby-git-hooks` in a Rails project, I'm in the `Rails.root + 'config'` directory and I issue `rails server`, it's going to fail.

Did you test for this?