rebase: fix docs about incompatibilities with --root

In commit 5dacd4abdd ("git-rebase.txt: document incompatible options",
2018-06-25), I added notes about incompatibilities between options for
the apply and merge backends.  Unfortunately, I inverted the condition
when --root was incompatible with the apply backend.  Fix the
documentation, and add a testcase that verifies the documentation
matches the code.

While at it, the documentation for --root also tried to cover some of
the backend differences between the apply and merge backends in relation
to reapplying cherry picks.  The information:
  * assumed that the apply backend was the default (it isn't anymore)
  * was written before --reapply-cherry-picks became an option
  * was written before the detailed information on backend differences
All of these factors make the sentence under --root about reapplying
cherry picks contradict information that is now available elsewhere in
the manual, and the other references are correct.  So just strike this
sentence.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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