Simplify FilteredRepository.FilteredRefDatabase#getAdditionalRefs()

Just return the list of refs from
git.getRefDatabase().getAdditionalRefs() instead of adding those refs to
a new Map and then return the Map values.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Ia2e1016c976c56a4ddd5267e18bd6decf6607bc3