darcs init
touch a
darcs add a
darcs rec -am 'a'
echo "line1" > a
darcs rev -a
echo "yn" | darcs ob
causes "darcs failed: Cancelled."
when the user has answered no to "This operation will make unrevert
impossible!" in obliterate, because "fail" is called within
"removeFromUnrevertContext" in src/Darcs/Repository/Internal.hs
I don't think this should be a failure - the user has told darcs to stop.