rebase: provide better error message for apply options vs. merge config

rebase: provide better error message for apply options vs. merge config

When config which selects the merge backend (currently,
rebase.autosquash=true or rebase.updateRefs=true) conflicts with other
options on the command line (such as --whitespace=fix), make the error
message specifically call out the config option and specify how to
override that config option on the command line.

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