resolve-undo: teach "update-index --unresolve" to use resolve-undo info

The update-index plumbing command had a hacky --unresolve implementation
that was written back in the days when merge was the only way for users to
end up with higher stages in the index, and assumed that stage #2 must
have come from HEAD, stage #3 from MERGE_HEAD and didn't bother to compute
the stage #1 information.

There were several issues with this approach:

 - These days, merge is not the only command, and conflicts coming from
   commands like cherry-pick, "am -3", etc. cannot be recreated by looking
   at MERGE_HEAD;

 - For a conflict that came from a merge that had renames, picking up the
   same path from MERGE_HEAD and HEAD wouldn't help recreating it, either;

 - It may have been Ok not to recreate stage #1 back when it was written,
   because "diff --ours/--theirs" were the only availble ways to review
   conflicts and they don't need stage #1 information.  "diff --cc" that
   was invented much later is a lot more useful way but it needs stage #1.

We can use resolve-undo information recorded in the index extension to
solve all of these issues.

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

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