Merge branch 'en/merge-recursive-debug'

Merge branch 'en/merge-recursive-debug'

Remove remnants of the recursive merge strategy backend, which was
superseded by the ort merge strategy.

* en/merge-recursive-debug:
  builtin/{merge,rebase,revert}: remove GIT_TEST_MERGE_ALGORITHM
  tests: remove GIT_TEST_MERGE_ALGORITHM and test_expect_merge_algorithm
  merge-recursive.[ch]: thoroughly debug these
  merge, sequencer: switch recursive merges over to ort
  sequencer: switch non-recursive merges over to ort
  merge-ort: enable diff-algorithms other than histogram
  builtin/merge-recursive: switch to using merge_ort_generic()
  checkout: replace merge_trees() with merge_ort_nonrecursive()