.arcconfig

tests: demonstrate crash when trying to rebase merge without its parents

As the test case shows, when "hg rebase -d G -r 'B + D + F'" is run on
the following graph, we crash with traceback. It's reasonable to fail
because we can not easily produce a correct rebased F. The problem is
what diff to apply to either the rebased B or the rebased D. We could
potentially produce the result by e.g. applying the (F-D) diff to the
rebased B and then applying the reverse (E-D) diff on top, but that
could result in merge conflicts in each of those steps, which we don't
have a way of dealing with. So for now, let's just add a test case to
demonstrate that we crash (i.e. the AssertionError is clearly
incorrect since the user can run into it).

  F
 /|
C E
| |
B D G
 \|/
  A

Differential Revision: https://phab.mercurial-scm.org/D212

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