backout: allow backout of merge changeset with --parent option.
--parent allows to choose which parent of merge to revert to.
#!/bin/sh
hg init a
cd a
hg init b
echo x > b/x
echo '# should print nothing'
hg st
echo '# should print ? b/x'
hg st b/x
hg add b/x
echo '# should print A b/x'
hg st
echo '# should forget b/x'
hg revert
echo '# should print nothing'
hg st b