fold: cleanly abort on empty fold set (issue5453) We now handle the empty set case, cleanly aborting instead of crashing with a traceback. The message used match the output of 'hg push' in similar situation.

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