Merge branch 'en/ort-directory-rename'

ORT merge strategy learns to infer "renamed directory" while
merging.

* en/ort-directory-rename:
  merge-ort: fix a directory rename detection bug
  merge-ort: process_renames() now needs more defensiveness
  merge-ort: implement apply_directory_rename_modifications()
  merge-ort: add a new toplevel_dir field
  merge-ort: implement handle_path_level_conflicts()
  merge-ort: implement check_for_directory_rename()
  merge-ort: implement apply_dir_rename() and check_dir_renamed()
  merge-ort: implement compute_collisions()
  merge-ort: modify collect_renames() for directory rename handling
  merge-ort: implement handle_directory_level_conflicts()
  merge-ort: implement compute_rename_counts()
  merge-ort: copy get_renamed_dir_portion() from merge-recursive.c
  merge-ort: add outline of get_provisional_directory_renames()
  merge-ort: add outline for computing directory renames
  merge-ort: collect which directories are removed in dirs_removed
  merge-ort: initialize and free new directory rename data structures
  merge-ort: add new data structures for directory rename detection

Author: gitster