Is Ref the right type here? Shouldn't we introduce a type which can hold lightweight and annotated tags. Like Shawn suggested in http://egit.eclipse.org/r/#patch,unified,4346,1,org.eclipse.jgit.test/tst/org/eclipse/jgit/api/TagCommandTest.java
If we don't introduce it then the user of this API has to care for the details (checking that the ref is under 'refs/tags', differentiate between the different types of tags). Since this belongs to the public API we should get it right in the first shot.