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.
% initialize repository
adding a
% try hgweb request
0
1f424bb22ec05c3c6bc866b6e67efe43 page1