branch heads: fix regression introduced in
e67e5b60e55f (
issue1726)
For merge nodes it is not adequate to only check a single possible
branch head for whether it is an ancestor of the latest head, but it
needs to be done for each possible branch head.
http://example.com/no/anchor, revs: None, checkout: None
http://example.com/an/anchor, revs: ['foo'], checkout: 'foo'
http://example.com/no/anchor/revs, revs: ['foo'], checkout: 'foo'
http://example.com/an/anchor/revs, revs: ['foo', 'bar'], checkout: 'foo'
http://example.com/an/anchor/rev-None, revs: ['foo'], checkout: 'foo'