[436246] CDOObject.cdoPrefetch() useless on CDOBranch

Have prefetch returning RevisionInfo, instead of referenced CDORevision,
to have PointCDORevision put in cache.
Fix the case of a PointerCDORevision without target by putting result as
target in RevisionInfo.readResult().

Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=436246
Change-Id: I49744c824b732da12fdbc7818d305d7f5d11ea00
Signed-off-by: Esteban Dugueperoux <esteban.dugueperoux@obeo.fr>