merge-ort: add implementation of both sides renaming differently

Implement rename/rename(1to2) handling, i.e. both sides of history
renaming a file and rename it differently.  This code replaces the
following from merge-recurisve.c:

  * all the 1to2 code in process_renames()
  * the RENAME_ONE_FILE_TO_TWO case of process_entry()
  * handle_rename_rename_1to2()

Also, there is some shared code from merge-recursive.c for multiple
different rename cases which we will no longer need for this case (or
other rename cases):

  * handle_file_collision()
  * setup_rename_conflict_info()

The consolidation of five separate codepaths into one is made possible
by a change in design: process_renames() tweaks the conflict_info
entries within opt->priv->paths such that process_entry() can then
handle all the non-rename conflict types (directory/file, modify/delete,
etc.) orthogonally.  This means we're much less likely to miss special
implementation of some kind of combination of conflict types (see
commits brought in by 66c62eaec6 ("Merge branch 'en/merge-tests'",
2020-11-18), especially commit ef52778708 ("merge tests: expect improved
directory/file conflict handling in ort", 2020-10-26) for more details).
That, together with letting worktree/index updating be handled
orthogonally in the merge_switch_to_result() function, dramatically
simplifies the code for various special rename cases.

To be fair, there is a _slight_ tweak to process_entry() here to make
sure that the two different paths aren't marked as clean but are left in
a conflicted state.  So process_renames() and process_entry() aren't
quite entirely orthogonal, but they are pretty close.

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

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