template: add successors template
The successors templates display the closest visible successors of each
changeset, we may have gap, like in this case:
A -> B -> C -> D
If we display A, C and D but not B, we can't display than B is the closest
successor of A because it's not displayed. We display C as the closest
successor of A instead.
test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t