[PATCH 00/20] fundamentals of merge-ort implementation

This series depends on a merge of en/strmap and en/merge-ort-api-null-impl.

The goal of this series is to show the new design and structure behind merge-ort, particularly the bits that are completely different to how merge-recursive operates. There are still multiple important codepaths that die with a "Not yet implemented" message, so the new merge algorithm is still not very usable (however, it can handle very trivial rebases or cherry-picks at the end of the series).

At a high level, merge-ort avoids unpack_trees() and the index, instead using traverse_trees() and its own data structure. After it is done processing each path, it writes a tree. Only after it has created a new tree will it touch the working copy or the index. It does so by using a simple checkout-like step to switch from head to the newly created tree. If there are unmerged entries, it touches up the index after the checkout-like step to record those higher order stages.

In the series:
  * Patch 1 adds some basic data structures.
  * Patch 2 documents the high-level steps.
  * Patches 3-5 are some simple setup.
  * Patches 6-10 collect data from the traverse_trees() operation.
  * Patches 11-15 process the individual paths and create a tree.
  * Patches 16-19 handle checkout-and-then-write-higher-order-stages.
  * Patch 20 frees data from the merge_options_internal data structure

Elijah Newren (20):
  merge-ort: setup basic internal data structures
  merge-ort: add some high-level algorithm structure
  merge-ort: port merge_start() from merge-recursive
  merge-ort: use histogram diff
  merge-ort: add an err() function similar to one from merge-recursive
  merge-ort: implement a very basic collect_merge_info()
  merge-ort: avoid repeating fill_tree_descriptor() on the same tree
  merge-ort: compute a few more useful fields for collect_merge_info
  merge-ort: record stage and auxiliary info for every path
  merge-ort: avoid recursing into identical trees
  merge-ort: add a preliminary simple process_entries() implementation
  merge-ort: have process_entries operate in a defined order
  merge-ort: step 1 of tree writing -- record basenames, modes, and oids
  merge-ort: step 2 of tree writing -- function to create tree object
  merge-ort: step 3 of tree writing -- handling subdirectories as we go
  merge-ort: basic outline for merge_switch_to_result()
  merge-ort: add implementation of checkout()
  tree: enable cmp_cache_name_compare() to be used elsewhere
  merge-ort: add implementation of record_unmerged_index_entries()
  merge-ort: free data structures in merge_finalize()

 merge-ort.c | 922 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 tree.c      |   2 +-
 tree.h      |   2 +
 3 files changed, 922 insertions(+), 4 deletions(-)