Fix for https://github.com/ktoso/maven-git-commit-id-plugin/issues/124

Hi konrad,
I just wrote a fix for https://github.com/ktoso/maven-git-commit-id-plugin/issues/124.
I thought it would be a good idea to support a Full-Path and a Relative-Path for the "generateGitPropertiesFilename"-Property.

Any suggestions for improvements are welcome ;)