perf: use the cache to compute the obsolete set. The official "obsolete" computation is switch to using the cache. This provide noticable speed for operation that does not need to actually access the obs markers. The part relating to obsolete changeset computation disappear from the execution profile when it is used.

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