builtin/merge-recursive: switch to using merge_ort_generic()

builtin/merge-recursive: switch to using merge_ort_generic()

Switch from merge-recursive to merge-ort.  Adjust the following
testcases due to the switch:

* t6430: most of the test differences here were due to improved D/F
  conflict handling explained in more detail in ef527787089c (merge
  tests: expect improved directory/file conflict handling in ort,
  2020-10-26).  These changes weren't made to this test back in that
  commit simply because I had been looking at `git merge` rather than
  `git merge-recursive`.  The final test in this testsuite, though, was
  expunged because it was looking for specific output, and the calls to
  output_commit_title() were discarded from merge_ort_internal() in its
  adaptation from merge_recursive_internal(); see 8119214f4e70
  (merge-ort: implement merge_incore_recursive(), 2020-12-16).

* t6434: This test is built entirely around rename/delete conflicts,
  which had a suboptimal handling under merge-recursive.  As explained
  in more detail in commits 1f3c9ba707 ("t6425: be more flexible with
  rename/delete conflict messages", 2020-08-10) and 727c75b23f ("t6404,
  t6423: expect improved rename/delete handling in ort backend",
  2020-10-26), rename/delete conflicts should each have two entries in
  the index rather than just one.  Adjust the expectations for all the
  tests in this testcase to see the two entries per rename/delete
  conflict.

* t6424: merge-recursive had a special check-if-toplevel-trees-match
  check that it ran at the beginning on both the merge-base and the
  other side being merged in.  In such a case, it exited early and
  printed an "Already up to date." message.  merge-ort got rid of
  this, and instead checks the merge base tree matching the other
  side throughout the tree instead of just at the toplevel, allowing
  it to avoid recursing into various subtrees.  As part of that, it
  got rid of the specialty toplevel message.  That message hasn't
  been missed for years from `git merge`, so I don't think it is
  necessary to keep it just for `git merge-recursive`, especially
  since the latter is rarely used.  (git itself only references it
  in the testsuite, whereas it used to power one of the three
  rebase backends that existed once upon a time.)

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