merge-ort: implement CE_SKIP_WORKTREE handling with conflicted entries

merge-ort: implement CE_SKIP_WORKTREE handling with conflicted entries

When merge conflicts occur in paths removed by a sparse-checkout, we
need to unsparsify those paths (clear the SKIP_WORKTREE bit), and write
out the conflicted file to the working copy.  In the very unlikely case
that someone manually put a file into the working copy at the location
of the SKIP_WORKTREE file, we need to avoid overwriting whatever edits
they have made and move that file to a different location first.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>