merge-ort: add implementation of rename collisions

Implement rename/rename(2to1) and rename/add handling, i.e. a file is
renamed into a location where another file is added (with that other
file either being a plain add or itself coming from a rename).  Note
that rename collisions can also have a special case stacked on top: the
file being renamed on one side of history is deleted on the other
(yielding either a rename/add/delete conflict or perhaps a
rename/rename(2to1)/delete[/delete]) conflict.

One thing to note here is that when there is a double rename, the code
in question only handles one of them at a time; a later iteration
through the loop will handle the other.  After they've both been
handled, process_entry()'s normal add/add code can handle the collision.

This code replaces the following from merge-recurisve.c:

  * all the 2to1 code in process_renames()
  * the RENAME_TWO_FILES_TO_ONE case of process_entry()
  * handle_rename_rename_2to1()
  * handle_rename_add()

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 six 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.

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

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