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:
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
a
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
a not present
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
a