rebase: mark --update-refs as requiring the merge backend
--update-refs is built in terms of the sequencer, which requires the
merge backend. It was already marked as incompatible with the apply
backend in the git-rebase manual, but the code didn't check for this
incompatibility and warn the user. Check and error now.
While at it, fix a typo in t3422...and fix some misleading wording
(most options which used to be am-specific have since been implemented
in the merge backend as well).
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>