t / t4101 / diff.2-3on commit t5505: use "for-each-ref" to test for the non-existence of references (2eb7a0e)
   1--- a/frotz
   2+++ b/frotz
   3@@ -1,2 +1,3 @@
   4 a
   5+c
   6 b
   7\ No newline at end of file