topic: makes split keep the topic
Refactor split to extract the preparation of the working directory in a
separate function.
This helps topic wrap this function to save topic before update and restore it
after preparing the working directory just before the split.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes