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

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>