- If the git config contained old entries for submodules that were
  removed (they are not cleaned from the config automatically),
  it would add submodules with empty "dir", which lead to infinite
  recursion

- When recursing into submodules, we need to prepend the paths from the
  git output with the relative path from the main repository

Change-Id: I0bc6a1973f4cdeb1c3f09c05cd7736c25471f72e
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>