obscache: extract code to update from new revision Code cleanup and prepare upstreaming. The make the code more likely to work with the key validation returning an iterator of revs to update the cache with.

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