tests/test-issue433

test-merge-tools: windows fixes

- /bin/cat does not exist, expect 'cat'
- non-interactive mode is not correctly detected in pysh, force it

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

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

hg parents -r 0 doesnotexist
true