Your <referencedatabase> element is not fully filled out. That element is the same type as a <database> element and takes the same attributes. Something like this:
- <target name="diff-against-hibernate">
<liquibase:diffDatabaseToChangeLog
databaseref="my-database"
classpathref="mypath.classpath"
diffTypes="tables,columns,views">
<referencedatabase driver="${liquibase.database.driver}" url="${liquibase.persistence.unit}" user="${liquibase.database.username}" password="${liquibase.database.password}"/>
<txt outputfile="${change_control}/dbupdate/hibernate/${liquibase.hibernate.changelog.file}"/>
</liquibase:diffDatabaseToChangeLog>
</target>
See if that helps.