obsdiscovery: add an option to disable all obsdiscovery On large repository, computing the obshashtree can we as slow as resending everything. We give the option for people to opt out.

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