Revert "Remove obsolete require_gnu_diff function"

Oops, function require_gnu_diff is still needed in two places.

This reverts commit 8cae4fc2213649e36e8f9a4cf21c28a82de3705c.

tests/test-lib.sh

@@ -5,6 +5,18 @@
 # in any medium, are permitted without royalty provided the copyright
 # notice and this notice are preserved.
 
+# FIXME: Requires a version of diff that understands "-u".
+
+require_gnu_diff() {
+    case "`diff --version 2> /dev/null`" in
+    *GNU*)
+	;;
+    *)
+	echo "This test requires GNU diff" >&2
+	exit 77
+    esac
+}
+
 require() {
     utility="$1"
     if type require_${utility} > /dev/null 2> /dev/null; then