copy-tracing: test case where we go across a changed/deleted merge
This case is currently broken in the commit centric case. So we add an official
case for it with a note about it being broken.
Fixing it will requires multiples change in code related to merge, commit and
copy tracing, so we introduce it beforehand for simplicity.
Also, I have been sitting on this test for 3 months so I would rather have it