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.

template: add precursors template The precursors templates display the closest visible precursors of each changeset, we may have gap, like in this case: A -> B -> C If we display A and C but not B, we can't display than B is the closest precursor of C because it's not displayed. We display A as the closest precursor of C instead. Add a new test file as we will need to generate theses gaps and modifying test-evolve-obshistory.t will lead to many unrelated lines changes.