copies: add test that makes both the merging csets dirty and run w/o error 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". Differential Revision: https://phab.mercurial-scm.org/D5961