Revert changeset c67920d78248.
It breaks 'hg merge && hg up -c REV' when files exist only in one of the
manifests, making hg throw a traceback like:
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "mq=" >> $HGRCPATH
hg init a
cd a
hg qnew first.patch
hg qnew first.patch
touch ../first.patch
hg qimport ../first.patch
exit 0