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superseded by the ort merge strategy.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()

builtin/rebase.c

@@ -1575,11 +1575,6 @@ int cmd_rebase(int argc,
 			    options.default_backend);
 	}
 
-	if (options.type == REBASE_MERGE &&
-	    !options.strategy &&
-	    getenv("GIT_TEST_MERGE_ALGORITHM"))
-		options.strategy = xstrdup(getenv("GIT_TEST_MERGE_ALGORITHM"));
-
 	switch (options.type) {
 	case REBASE_MERGE:
 		options.state_dir = merge_dir();