tests: demonstrate another failure with in-memory rebase and copies This is a similar to dd1ab72be983 (test: demonstrate crash with in-memory rebase and copies, 2019-03-14). The new failure started with 57203e0210f8 (copies: calculate mergecopies() based on pathcopies(), 2019-04-11). It happens in the call to mergemod.update() on rebase.py:1268 where we call mergemod.update() to graft a node. Since the mergecopies() rewrite, that calls _related() with the filectx from the overlaywctx instead of a filectx from the changectx where the file was last modified. Either should be fine, so I don't think that's a bug. Differential Revision: https://phab.mercurial-scm.org/D6357