tests: demonstrate that rename is followed to wrong parent from merge
This test case shows another way that copies are handled differently
between `hg st` (pathcopies()) and `hg co -m` (mergecopies). The
reason is that pathcopies() calls _tracefiles(), which checks that the
file nodeid of an ancestor matches the file nodeid in the base
commit. mergecopies() should probably be doing the same.
Differential Revision: https://phab.mercurial-scm.org/D6323