[PATCH 2 of 2] add `allparents' item

[Paolo Bonzini]

> A comment on the comment: what is an "item"?  It looks to me like the
> proposed 'allparents' is an option common to many commands.  If I'm
> correct, you should probably refer to it as the "--allparents option".

No, it's not an option.  It's something that you can use in a template.

> Also, I think the notion of "at most two parents" is pretty hardwired
> in the guts of Mercurial.  So it would be equally accurate, and
> possibly clearer, to make this --bothparents.

Yes, that's a good idea.

Paolo