dokku rebuild handles repos with submodules

As mentioned in scottatron/dokku-rebuild#7 ```dokku rebuild``` did not update\include submodules in application.

This fix is just a port of original dokku git command implementation from [progrium/dokku/plugins/git/commands](https://github.com/progrium/dokku/blob/acb226cb86bfb8cf4570485711421e01c1a06739/plugins/git/commands#L4)