rebase: fix an incompatible-options error message

When the user specifies the apply backend with options that only work with the merge backend, such as git rebase --apply --exec /bin/true HEAD~3 the error message has always been fatal: --exec requires an interactive rebase This error message is misleading and was one of the reasons we renamed the interactive backend to the merge backend. Update the error message to state that these options merely require use of the merge backend. Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>