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.