merge-ort: add implementation of record_conflicted_index_entries()

merge-ort: add implementation of record_conflicted_index_entries()

After checkout(), the working tree has the appropriate contents, and the
index matches the working copy.  That means that all unmodified and
cleanly merged files have correct index entries, but conflicted entries
need to be updated.

We do this by looping over the conflicted entries, marking the existing
index entry for the path with CE_REMOVE, adding new higher order staged
for the path at the end of the index (ignoring normal index sort order),
and then at the end of the loop removing the CE_REMOVED-marked cache
entries and sorting the index.

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