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
HG=hg
"$HG" init
mkdir b
echo 'Bouncy' >b/bouncy
echo 'tricycle' >b/vehicle
"$HG" add b/bouncy
"$HG" add b/vehicle
"$HG" commit -m 'Adding bouncy'
echo 'bouncy' >>b/bouncy
"$HG" commit -m 'Making it bouncier'
"$HG" update -C 0
echo 'stationary' >>b/vehicle
"$HG" commit -m 'Clarifying the vehicle.'
"$HG" update -C 1
chmod a-w b/vehicle
"$HG" merge 2 2>&1 | sed 's|^\(.*[ ]\).*/\([^/]*/[^/]*/[^/]*\)$|\1\2|g'