t / t4101 / diff.1-2on commit xdiff: generate "anti-diffs" aka what is common to two files (a9ed376)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,3 +1,2 @@
   4 a
   5-b
   6-c
   7+b
   8\ No newline at end of file