tests/test-issue433

fix test-mq-eol under --pure (mimic diffhelper.c behaviour)

bug discovered by apycot (apycot.hg-scm.org)

#!/bin/sh
# http://mercurial.selenic.com/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true