[564680] Improved diagram display.

When merging a deletion of a Sirius element, the diagrams disappear from
the content merge viewer and are replaced by a white background. This is
due to the DiagramMatchAccessorImpl#getDiagram(MergeViewerSide) method,
which did not correctly find the diagram associated with a deleted
graphical element.

Bug: 564680
Change-Id: I48151683490f9617b661d7ae703bd4058838a30f
Signed-off-by: Glenn Plouhinec <glenn.plouhinec@obeo.fr>
21 files changed