New public Graph.revision_relation method

[Andrew Bennetts]

Martin von Gagern wrote:
[...]
> I propose to add a method Graph.revision_relation (no plural) that does
> a two-way comparison, and to also have global constants for the possible
> return values. Both Branch._revision_relations and Graph.is_ancestor

Sounds reasonable to me.  (As you probably guessed from IRC ;)

I think maybe “find_relation” would be a better name.  It's at least
superficially more consistent with other Graph methods, and it's a verb phrase
rather than a noun phrase.  I don't feel very strongly about it though.

-Andrew.