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:
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% push should push to default when default-push not set
pushing
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
% push should push to default-push when set
pushing
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files