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.