Fix NameRevCommand when repo has no annotated tags

Fix NameRevCommand when repo has no annotated tags

Previously, calling addAnnotatedTags() did not modify any state when
there were no annotated tags in the repository. This caused the code
to assume no addFoo() methods had been called, and fell back to the
default of adding refs/*. Instead, use null to indicate neither
addRefs() nor addAnnotatedTags() was called.

Add a test for this behavior.

Change-Id: I9926e5ac17e1a983cd399798993031c72bd79c2c