merge-ort: implement check_for_directory_rename()

merge-ort: implement check_for_directory_rename()

This is copied from merge-recursive.c, with minor tweaks due to using strmap
API and the fact that it can use opt->priv->paths to get all pathnames that
exist instead of taking a tree object.

This depends on a new function, handle_path_level_conflicts(), which
just has a placeholder die-not-yet-implemented implementation for now; a
subsequent patch will implement it.

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