merge-ort: add some high-level algorithm structure

merge-ort: add some high-level algorithm structure

merge_ort_nonrecursive_internal() will be used by both
merge_inmemory_nonrecursive() and merge_inmemory_recursive(); let's
focus on it for now.  It involves some setup -- merge_start() --
followed by the following chain of functions:

  collect_merge_info()
    This function will populate merge_options_internal's paths field,
    via a call to traverse_trees() and a new callback that will be added
    later.

  detect_and_process_renames()
    This function will detect renames, and then adjust entries in paths
    to move conflict stages from old pathnames into those for new
    pathnames, so that the next step doesn't have to think about renames
    and just can do three-way content merging and such.

  process_entries()
    This function determines how to take the various stages (versions of
    a file from the three different sides) and merge them, and whether
    to mark the result as conflicted or cleanly merged.  It also writes
    out these merged file versions as it goes to create a tree.

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