To use it you should place the plugin-sdk-differ-1.0-SNAPSHOT.jar file into the ${plugin-sdk.home}/lib directory and add the snippet to your build.xml

<target name="build-diff">
	<java classname="com.liferay.portal.tools.UpgradeDiffer" fork="true" newenvironment="true">
		<classpath refid="lib.classpath" />
		<classpath refid="portal.classpath" />
		<arg value="${basedir}" />
		<arg value="${source.zip}" />
	</java>
</target>

Enjoy!