octopus: allow criss-cross and clarify the message when it rejects

octopus: allow criss-cross and clarify the message when it rejects

We rejected multi-base merge situations even though we used the
same underlying multi-base git-read-tree as the resolve strategy
uses.  This was unneeded and did not add much to ensure the
merge to be truly trivial, so remove this restriction and be
more similar to what resolve does.

Also when the merge did not trivially resolve, we rejected
without stating that octopus strategy does not handle the
situation.

Signed-off-by: Junio C Hamano <junkio@cox.net>