[447231] DifferenceGroup: don't build a sub tree if the match is null

Bug: 447231
Change-Id: I84da495de92a017c960f08ae7de0db1e234c8ba8
Signed-off-by: Axel Richard <axel.richard@obeo.fr>
Simplification of DependencyData

Use already existing 'getTreeNodes(diff)' method from group providers
instead of create a new cross referencer.

Change-Id: Ia53417dadb9bc7fa9c506edadc38922222499409
Signed-off-by: Axel Richard <axel.richard@obeo.fr>