t/: new helper for tests that pass with ort but fail with recursive

There are a number of tests that the "recursive" backend does not handle
correctly but which the redesign in "ort" will.  Add a new helper in
lib-merge.sh for selecting a different test expectation based on the
setting of GIT_TEST_MERGE_ALGORITHM, and use it in various testcases to
document which ones we expect to fail under recursive but pass under
ort.

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

t/lib-merge.sh

@@ -0,0 +1,13 @@
+# Helper functions used by merge tests.
+
+test_expect_merge_algorithm () {
+	status_for_recursive=$1 status_for_ort=$2
+	shift 2
+
+	if test "$GIT_TEST_MERGE_ALGORITHM" = ort
+	then
+		test_expect_${status_for_ort} "$@"
+	else
+		test_expect_${status_for_recursive} "$@"
+	fi
+}