[MERGE] Merge now warns when it encounters a criss-cross

[Alexander Belchenko]

Martin Pool пишет:
> Martin Pool has voted tweak.
> Status is now: Conditionally approved
> Comment:
> If you think this will help user understanding it's ok with me.  Maybe 
> we can come up with a better message though:
> 
> +                warning('Warning: criss-cross merge encountered.')
> 
> So ok, the user now feels warned.  What are they to do?  If you could 
> tell them what they should do, or what the consequences are, or how to 
> avoid it, that would be better.

IIUC, we should suggest to use `merge --weave` as Aaron recently explain
in my question, see recent thread with title like "merge A B and merge B 
A produce different set of conflicts".