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 "fetch=" >> $HGRCPATH
hg init a
echo a > a/a
hg --cwd a commit -d '1 0' -Ama
hg clone a b
hg clone a c
echo b > a/b
hg --cwd a commit -d '2 0' -Amb
hg --cwd a parents -q
echo % should pull one change
hg --cwd b fetch ../a
hg --cwd b parents -q
echo c > c/c
hg --cwd c commit -d '3 0' -Amc
hg --cwd c fetch -d '4 0' -m 'automated merge' ../a
ls c