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>