merge-ort: add outline for computing directory renames

merge-ort: add outline for computing directory renames

Port some directory rename handling changes from merge-recursive.c's
detect_and_process_renames() to the same-named function of merge-ort.c.
This does not yet add any use or handling of directory renames, just the
outline for where we start to compute them.  Thus, a future patch will
add port additional changes to merge-ort's detect_and_process_renames().

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