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.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes