tests: add expected-failing test for minor subopimality

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

tests/Makefile.am

@@ -14,6 +14,7 @@ TESTS = \
   function-line-vs-leading-space \
   ignore-matching-lines \
   label-vs-func	\
+  large-subopt \
   new-file \
   no-dereference \
   no-newline-at-eof \
@@ -22,8 +23,12 @@ TESTS = \
   filename-quoting \
   colors
 
+XFAIL_TESTS = large-subopt
+
 EXTRA_DIST = \
-  $(TESTS) init.sh t-local.sh envvar-check
+  $(TESTS) init.sh t-local.sh envvar-check \
+  large-subopt.in1 \
+  large-subopt.in2
 
 # Note that the first lines are statements.  They ensure that environment
 # variables that can perturb tests are unset or set to expected values.

tests/large-subopt

@@ -0,0 +1,25 @@
+#!/bin/sh
+# This test is expected to fail at least with diffutils-3.6.
+# Demonstrate how diff can produce suboptimal output.
+# With these two files, diff -u prints output including this:
+# -2
+# +L: 361
+# +L: 361
+# +2
+# The trouble is that "2" line that is both added and removed.
+# This smaller patch could induce the same change:
+# +L: 361
+# +L: 361
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+diff -u \
+  "$abs_top_srcdir/tests/large-subopt.in1" \
+  "$abs_top_srcdir/tests/large-subopt.in2" \
+  | perl -n0 -e \
+     '/\n-2\n(\+L: 361\n){2}\+2\n/ and do {$e=1; last}; END{exit !$e}' \
+     && fail=1
+
+Exit $fail