In some unusual cases, diff -u prints suboptimal output.
* tests/large-subopt: New test script.
* tests/Makefile.am (TESTS): Add it.
(XFAIL_TESTS): Add it here, too, to record that this test is
currently expected to fail.
* tests/large-subopt.in1, tests/large-subopt.in2: Inputs derived from
those in http://bugs.gnu.org/28796
