resolve window: add "diff to common ancestor" buttons
When I resolve a conflict, I find it useful to have the diff against the common ancestor to better see what was changed.
To explain better see the graph.png attachement.
When I would merge these 2 branches and I get a conflict on readme.txt, I would want these 2 buttons
diff of rev10 to rev 5 on default for readme.txt (all changes to readme.txt done on default branch since common ancestor)
diff of rev9 to rev 5 on branchX for readme.txt (all changes to readme.txt done on branchX since common ancestor)