merge-ort: implement handle_path_level_conflicts()

merge-ort: implement handle_path_level_conflicts()

This is copied from merge-recursive.c, with minor tweaks due to:
  * using strmap API
  * merge-ort not using the non_unique_new_dir field, since it'll
    obviate its need entirely later with performance improvements
  * adding a new path_in_way() function that uses opt->priv->paths
    instead of doing an expensive tree_has_path() lookup to see if
    a tree has a given path.

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