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

tests/test-rebase-obsolete.t

@@ -473,6 +473,27 b' Test multiple root handling'
   
   $ cd ..
 
+Detach both parents
+
+  $ hg init double-detach
+  $ cd double-detach
+
+  $ hg debugdrawdag <<EOF
+  >   F
+  >  /|
+  > C E
+  > | |
+  > B D G
+  >  \|/
+  >   A
+  > EOF
+
+BROKEN: This raises an exception
+  $ hg rebase -d G -r 'B + D + F' 2>&1 | grep '^AssertionError'
+  AssertionError: no base found to rebase on (defineparents called wrong)
+
+  $ cd ..
+
 test on rebase dropping a merge
 
 (setup)