This series of patches is to cover a case in fullcopytracing algorithms
where both the merging csets are not descendant of merge base.
In this algorithm we call a merging cset "dirty" if that cset is not the
descendant of merge base. That said, added test in this patch cover case
when both the merging csets are "dirty".
Actually this case of "both dirty" was encountered by Pulkit when he was
working on content-divergence where it is possible that both the csets
are not descendant of merging base.
For reference you can look into: https://phab.mercurial-scm.org/D3896
As this test run fine without any error and correctly traced the copies, I
added this test to make sure that it doesn't break even after I will modify
some code in next patches to fix an error. Next patch adds the tests where
this algorithm throws an error for the same case of "both dirty".