merge-ort: handle book-keeping around two- and three-way content merge

merge-ort: handle book-keeping around two- and three-way content merge

In addition to the content merge (which will go in a subsequent commit),
we need to worry about conflict messages, placing results in higher
order stages in case of a df_conflict, and making sure the results are
placed in ci->merged.result so that they will show up in the working
tree.  Take care of all that external book-keeping, moving the
simplistic just-take-HEAD code into the barebones handle_content_merge()
function for now.  Subsequent commits will flesh out
handle_content_merge().

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