fix maven-release-plugin not committing changed pom.xml when using git repository

With one of my projects I just had the problem that the maven-release-plugin did not work as expected: I did a mvn release:prepare, answered the questions about the version number of the release and the next snapshot version, but the release plugin did not work as expected: The version in the pom.xml was updated to the release version, the pom.xml was added to git stage, but not committed. After that, the new tag was created on the version that had still the pom.