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 init
echo foo > a
echo foo > b
hg add a b
hg ci -m "test" -d "1000000 0"
echo blah > a
hg ci -m "branch a" -d "1000000 0"
hg co 0
echo blah > b
hg ci -m "branch b" -d "1000000 0"
HGMERGE=true hg merge 1
hg ci -m "merge b/a -> blah" -d "1000000 0"
hg co 1
HGMERGE=true hg merge 2
hg ci -m "merge a/b -> blah" -d "1000000 0"
hg log
hg debugindex .hg/00changelog.i
echo
echo 1
hg manifest 1
echo 2
hg manifest 2
echo 3
hg manifest 3
echo 4
hg manifest 4
echo
hg debugindex .hg/data/a.i
hg verify