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:
#header#<title>Mercurial Error</title></head><body><h2>Mercurial Error</h2><p>An error occured while processing your request:</p><p>#error|escape#</p>#footer#