From: Elijah Newren <newren@gmail.com>

--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 warn now.

While at it, fix a typo in t3422...and fix some misleading wording (all
useful options other than --whitespace=fix have long since been
implemented in the merge backend).

Signed-off-by: Elijah Newren <newren@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1466.git.1674106587550.gitgitgadget@gmail.com