test-copies: add test chaining multiple merges
Right now, the copy tracing logic take the right decision for merges, but it
does not keep track of the right information about these decision and can fall
into later traps. We start with highlighting this possibility by adding new
tests, and we will fix them later.
Check the inline test documentation for details.
Differential Revision: https://phab.mercurial-scm.org/D9609