evolve: exclude "identical" precursors from the precursors set
I'm not sure it is the right thing to do, but it simply thing a lot for now.
Otherwise, using 'precursors' to find rewind direction become too complex.
This is going to give problematic result in case of prune. Since there are no
other precursors to follow.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes
^docs/training/graphs/
^docs/training/html/
^docs/training/index.html
^docs/training/graphviz-images/
^docs/training/img/
^docs/training/output/
^docs/training/pandocfilters/