Load git repo from projectDir instead of current working directory

If the gradle project using the gitrelease plugin is not accessed from the
command line inside the gradle project it fails with an exception as it
cannot figure out what the git repository is.

This fix uses the projects projectDir as a starting point in finding the
git repository. This might cause issues if someone relied on this feature
and used the gitrelease plugin without having the working directory being
the projects checked out git repository.

This fix allows gradle projects using the gitrelease to be imported
into IntelliJ directly with the built-in gradle support.