Also look in DRUPAL_ROOT.'/.git/modules' for submodule .git dir
[gh-809] Don't hardcode, look in .git file for gitdir
[gh-809] Remove use of $success
[gh-809] Need to be relative to dirname($gitPath)
Merge pull request #809 from ericras/submodule-version-detection

Also look in DRUPAL_ROOT.'/.git/modules' for submodule .git dir