discovery: filter null from the discovery revset If there is nothing in common, common will be "null" and the revset will now returns "-1" crashing the discovery process. We filter out the null revision to prevent that.

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