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Remove remnants of the recursive merge strategy backend, which was superseded by the ort merge strategy.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