tests: add test for latest fix

* NEWS (Bug fixes): Describe it.
* tests/invalid-re: New test for just-fixed bug.
* tests/Makefile.am (TESTS): Add it.

NEWS

@@ -4,6 +4,12 @@ GNU diffutils NEWS                                    -*- outline -*-
 
 ** Bug fixes
 
+  Using an invalid regular expression with --ignore-matching-lines=RE (-I)
+  no longer causes stack overflow.  Before, with an invocation like the
+  following, diff would diagnose the error, but would still proceed to
+  blow the stack: diff -Ia -I\\ <(echo) <(echo b)
+  [bug introduced in 2.9]
+
   diff no longer mishandles line numbers exceeding 2**31 on Mingw-w64.
 
   the ---presume-output-tty (ostensibly test-only) option would cause

tests/Makefile.am

@@ -9,6 +9,7 @@ TESTS = \
   diff3 \
   excess-slash \
   help-version	\
+  invalid-re	\
   function-line-vs-leading-space \
   ignore-matching-lines \
   label-vs-func	\

tests/invalid-re

@@ -0,0 +1,18 @@
+#!/bin/sh
+# Ensure we diagnose and fail upon first invalid -I regex.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+
+fail=0
+
+echo > a || framework_failure_
+echo b > b || framework_failure_
+echo 'diff: \: Trailing backslash' > exp-err || framework_failure_
+
+# This must fail with an exit status of 2:
+returns_ 2 diff -Ix -I\\ a b > out 2> err || fail=1
+
+compare /dev/null out || fail=1
+compare exp-err err || fail=1
+
+Exit $fail