resolve-undo: "checkout -m path" uses resolve-undo information

resolve-undo: "checkout -m path" uses resolve-undo information

Once you resolved conflicts by "git add path", you cannot recreate the
conflicted state with "git checkout -m path", because you lost information
from higher stages in the index when you resolved them.

Since we record the necessary information in the resolve-undo index
extension these days, we can reproduce the unmerged state in the index and
check it out.

Signed-off-by: Junio C Hamano <gitster@pobox.com>