tests: demonstrate another failure with in-memory rebase and copies

Authored by martinvonz.

Description

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

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