git cannot run submodule if the .git folder is not at the project root

I have a grails project that must defines a project under a child folder of the git repository root.

- repo root
   - .git
   - grails-project-root
      - .projectile
   - ... other top level folders ...

`projectile-project-vcs` correctly locates the git repository at 

`repo root`

 via `projectile-locate-dominating-file` prompting `projectile-get-sub-projects-command` to return the git submodule command string to be run at 

`grails-project-root`.

When you try and do this in git, it fails and returns

> You need to run this command from the toplevel of the working tree.

This request alleviates this by making sure that the `.git` folder is in
`projectile-project-root` before returning the submodule string.