Merge branch 'en/merge-ort-impl'

The merge backend "done right" starts to emerge.

* en/merge-ort-impl:
  merge-ort: free data structures in merge_finalize()
  merge-ort: add implementation of record_conflicted_index_entries()
  tree: enable cmp_cache_name_compare() to be used elsewhere
  merge-ort: add implementation of checkout()
  merge-ort: basic outline for merge_switch_to_result()
  merge-ort: step 3 of tree writing -- handling subdirectories as we go
  merge-ort: step 2 of tree writing -- function to create tree object
  merge-ort: step 1 of tree writing -- record basenames, modes, and oids
  merge-ort: have process_entries operate in a defined order
  merge-ort: add a preliminary simple process_entries() implementation
  merge-ort: avoid recursing into identical trees
  merge-ort: record stage and auxiliary info for every path
  merge-ort: compute a few more useful fields for collect_merge_info
  merge-ort: avoid repeating fill_tree_descriptor() on the same tree
  merge-ort: implement a very basic collect_merge_info()
  merge-ort: add an err() function similar to one from merge-recursive
  merge-ort: use histogram diff
  merge-ort: port merge_start() from merge-recursive
  merge-ort: add some high-level algorithm structure
  merge-ort: setup basic internal data structures