New public Graph.revision_relation method

[Aaron Bentley]

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Martin von Gagern wrote:
> Hi!
> 
> I'm looking for a public interface to cheaply test the relation between
> two revisions, i.e. "a is an ancestor of b", "b is an ancestor of a" or
> "neither is an ancestor of the other".

We use Graph.heads for this.

If a is an ancestor of b, then Graph.heads will return b.
If b is an ancestor of a, then Graph.heads will return a.
If neither is an ancestor of the other, then Graph.heads will return both.

Of course, it may simplify the client API to provide a revision_relation
method, but if you do, I suggest basing it on heads and using the logic
described above.

Aaron
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