merge-tree: provide easy access to `ls-files -u` style info

merge-tree: provide easy access to `ls-files -u` style info

Much like `git merge` updates the index with information of the form
    (mode, oid, stage, name)
provide this output for conflicted files for merge-tree as well.
Provide a --name-only option for users to exclude the mode, oid, and
stage and only get the list of conflicted filenames.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>