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.
#!/bin/sh
hg init
echo a > a
hg ci -Am0
touch .hg/store/journal
echo foo > a
hg ci -Am0
hg recover